Even though I have the latest Adobe Flash player 11.2 for Linux in Firefox - ref: Flash player security doubts - a particular online flash game still requests that I update to the latest flash player.
I've tried this online game on my laptop (Windows 8 with Flash Player 11.8) and it has no problems.
Is there a way to trick Ubuntu or this game into thinking Flash Player 11.8 is "installed"?
Note: This answer is severely outdated. Please refer to karel’s answer instead which describes how Canonical and Adobe currently recommend to install Flash Player in Ubuntu.
For Linux, the latest version of Adobe Flash Player is 11.2 which you already have. Sorry, but it cannot be updated to 11.8. Adobe provides security backports to Flash Player 11.2 for Linux.
To install the Flash plug-in on Ubuntu 13.04 (Raring Ringtail), please follow this process:
Enable the multiverse repository, as shown here: How do I enable the "multiverse" repository?
Open a terminal window (press Ctrl+Alt+T) and copy/paste this line:
sudo apt-get install flashplugin-installer
When the Flash Player is installed, close the terminal window and restart your browser.
Flash player is completely dead for all operating systems as of 2021.
To install the latest version of flash player search the Dash (in Ubuntu releases before 17.10) or the Show Applications dashboard (in Ubuntu 17.10 or later) for Software & Updates and open the Software & Updates window. Click the Other Software tab in the Software & Updates window, and put a check mark in the checkbox to the left of where it says: Canonical Partners.
Software & Updates in Ubuntu 17.10
Click the Close button to close the Software & Updates window.
Open the terminal and type:
sudo apt-get update
sudo apt-get install adobe-flashplugin
sudo apt install browser-plugin-freshplayer-pepperflash # 16.04 and later
Flash plugin for Linux provided by Adobe stopped at version 11.2. For Chrome/Chromium users there is Pepper Flash plugin, but it's not supported by Firefox/Iceweasel/other browsers. In Ubuntu 16.04 and later browser-plugin-freshplayer-pepperflash from the default Ubuntu repositories allows one to use the Pepper Flash plugin from Chrome in Firefox and any other web browser supporting NPAPI plugins. It works better than adobe-flashplugin in Firefox.
Adobe no longer provides Flash Player updates for Linux except for security backports.
From the Abobe Flash Blog:
For Flash Player releases after 11.2, the Flash Player browser plugin for Linux will only be available via the “Pepper” API as part of the Google Chrome browser distribution and will no longer be available as a direct download from Adobe. Adobe will continue to provide security updates to non-Pepper distributions of Flash Player 11.2 on Linux for five years from its release.
You can still use the latest flash in Google Chrome or Chromium 
with the 'Pepper Flash' plugin
Adobe Flash Player is directly integrated with Google Chrome and enabled by default. Available updates for Adobe Flash Player are automatically included in Chrome system updates.
You will need to install it manually in Chromium, though, see here
Sadly, Adobe has dropped support for the GNU/Linux version of Flash Player (though security updates are still being provided). However, there are still two ways of getting newer versions.
The first is to run the Windows version through Wine, a software emulation layer designed to make Windows software work on GNU/Linux and other Unix-like systems. You'll need a Windows web browser (such as the Windows version of Firefox), with the Windows version of Flash Player.
Or, you could install Google Chrome, as it always has a recent version of Flash, even on Ubuntu. If you choose Chrome, you won't need Wine. This may no longer be an option. See update #3.
Update: I now know of a third way to do this: Pipelight! Pipelight was originally a browser plugin meant to use a fork of WINE to run Microsoft Silverlight. However, at some point, the developers decided to add support for Flash as well. See Here for instructions on how to install Pipelight, and enable Flash Player.
However, this method isn't perfect; if you find that your browser won't respond to your mouse, you may either switch workspaces (using you desktop's keyboard shortcuts), or switch windows (again keyboard shortcuts). Either way, when you switch back, you should be able to click again. Please note, I've only tested this with Linux Mint's Cinnamon desktop (which is forked from Gnome), and can't guarantee this will work on other desktops. If all else fails, you can switch to tty, and kill pipelight using pkill pluginloader.exe.
Also, you might want to run sudo pipelight-plugin --update from time to time, so that Pipelight will know to install an updated version of flash player. This both prevents Pipelight from trying to download plugins from dead links, and ensures said plugins stay up-to-date. Or, you can create a cron file to run the command automatically. To do this, run sudo bash -c 'echo -e \#\!"/bin/bash\n\npipelight-plugin --update" > /etc/cron.weekly/pipelight-update; chmod a+x /etc/cron.weekly/pipelight-update' This will allow your Pipelight's list of plugins to be updated weekly, although the actuall plugins won't be updated untill you start your NPAPI-based browser.
Update 2: I found another plugin which uses Pepper (Google Chrome) Flash Player inside other browsers (such as firefox). This plugin, known as freshplayerplugin, is a native version of Flash, so no WINE is required. Please note: although I haven't tried this method, Pepper Flash is known to have problems with DRM-Protected videos, such as those found on Amazon Prime. If you watch DRM-protected videos with Flash Player, you might want to use Pipelight.
The above link will tell you how install FreshPlayerPlugin by cloning a git repository and compiling the code yourself. Or, you can install the pepflashplugin-installer package from the skunk/pepper-flash ppa: sudo add-apt-repository ppa:skunk/pepper-flash && sudo apt-get update && sudo apt-get install pepflashplugin-installer. See the bottom of THIS page for instructions on how to enable Chromium to use pepperflash. Warning: this depends on Google Chrome support. Please see update #3.
Update 3: Google has dropped Chrome support for all 32-bit GNU/Linux systems. If you have a 64-bit system, you can use Chrome as usual. Otherwise, you will have to either run an old version of Chrome (good luck finding one. Also, do be warned that old browsers are not secure), switch to a 64-bit system, or attempt to run the Windows version through WINE. Because 32-bit systems are no longer supported, you can no longer use the method described in update #2 on 32-bit systems.
Update 4: Pipelight has been discontinued by the author. You can no longer install Flash Player with it. However, Adobe has decided to bump the GNU/Linux version of Flash player to the latest versions, so I guess you don't need pipelight or freshplayerplugin. in fact, that kind of makes this entire answer obsolete.
Select your architecture: Linux (32-bit) or Linux (64-bit).
uname -i: if it returns something having the number 64 in it, your system is 64-bit. Otherwise, it's 32-bit.Download the NPAPI tarball: select the option having (.tar.gz) - NPAPI in it and then click onto the "Download now" button.
tarball.tar.gz, tarball.tar.zip, tarball.tar.7z and tarball.tar.bz2 are all examples of tarballs.Save the tarball into the /tmp folder with the name flash.tar.gz. If your browser automatically downloads the tarball, thus making it impossible for you to rename the tarball before the download starts, wait for the download to end, then go to the folder where the tarball's been put, rename the tarball to flash.tar.gz and then move it to /tmp.
Start the shell terminal and then run this command:
cd /tmp ; tar -xvf flash.tar.gz
Now run this supercommand in order to place the Adobe Flash plugin in the system folder:
if ! [ -d /usr/lib/adobe-flashplugin/ ]; then sudo mkdir /usr/lib/adobe-flashplugin ; else echo /usr/lib/adobe-flashplugin/ already exists ; fi ; sudo cp -f libflashplayer.so /usr/lib/adobe-flashplugin/ ; sudo ln -sf /usr/lib/adobe-flashplugin/libflashplayer.so /usr/lib/flashplugin-installer/libflashplayer.so
Then run this command in order to place the Adobe Flash folders in their respective locations:
sudo cp -rf /tmp/usr /
You can now run your Mozilla/Gecko browser (Firefox, Iceweasel or SeaMonkey) and then access the URL about:plugins in order to check if your browser has detected the flash plugin located at /usr/lib/adobe-flashplugin/libflashplayer.so (it should be also visible by accessing about:addons and then the Plugins section). If you see something like Shockwave Flash 24.0 r0, then go to http://www.adobe.com/software/flash/about in order to check if the flash applet detects your NPAPI flash install and tells you which version you're currently using.
/usr/lib/flashplugin-installer/libflashplayer.so instead of /usr/lib/adobe-flashplugin/libflashplayer.so. If it happens, ignore it: it's normal.Restart your computer just to make sure that the applications menu shows your brand-new "Adobe Flash Player" control panel. If it doesn't, you should be able to start it by running this shell command:
/usr/bin/flash-player-properties
Note: if you're using the 32-bit SeaMonkey on a 64-bit system, the browser won't "see" the plugin. Use another browser (e.g. Firefox).
Open a shell terminal window and run this supercommand:
sudo rm -r /usr/lib/adobe-flashplugin /usr/lib/flashplugin-installer/libflashplayer.so /usr/bin/flash-player-properties /usr/lib/kde4/kcm_adobe_flash_player.so /usr/lib64/kde4/kcm_adobe_flash_player.so /usr/share/applications/flash-player-properties.desktop /usr/share/kde4/services/kcm_adobe_flash_player.desktop /usr/share/pixmaps/flash-player-properties.png ; cd /usr/share/icons/hicolor ; sudo find . -name "flash-player-properties.png" -delete
Select your architecture: Linux (32-bit) or Linux (64-bit).
uname -i: if it returns something having the number 64 in it, your system is 64-bit. Otherwise, it's 32-bit.Download the PPAPI tarball: select the option having (.tar.gz) - PPAPI in it, and then hit the "Download now" button.
tarball.tar.gz, tarball.tar.zip, tarball.tar.7z and tarball.tar.bz2 are all examples of tarballs.Save the tarball into the /tmp folder with the name pepflash.tar.gz. If your browser automatically downloads the tarball, thus making it impossible for you to rename the tarball before the download starts, wait for the download to end, then go to the folder where the tarball's been put, rename the tarball to pepflash.tar.gz and then move it to /tmp.
Start the shell terminal and then run this command:
cd /tmp ; tar -xvf pepflash.tar.gz
Now run these commands in order to place the Adobe Flash plugin in the system folder:
if ! [ -d /usr/lib/adobe-flashplugin/ ]; then sudo mkdir /usr/lib/adobe-flashplugin ; else echo /usr/lib/adobe-flashplugin/ already exists ; fi ; sudo cp -f libpepflashplayer.so manifest.json /usr/lib/adobe-flashplugin/
You can now run your WebKit / Opera browser (only Opera versions prior to version 45), then access the URL about:plugins in order to check if your browser has detected the flash plugin located at /usr/lib/adobe-flashplugin/libpepflashplayer.so. Don't forget to click the Show details button (upper right) in order to expand the information fields and show extra info about each detected plugin. If you see something like Adobe Flash Player located at /usr/lib/adobe-flashplugin/libpepflashplayer.so, then go to http://www.adobe.com/software/flash/about just to make sure that the flash applet detects your PPAPI flash install and tells you which version you're currently using. If your browser is Opera version 45+ (version 45 or later), then the only way to check if the plugin is properly installed and functional is by accessing http://www.adobe.com/software/flash/about. If the flash applet detects your PPAPI flash install, then flash is working on your Opera 45+ browser.
Open a shell terminal window and run this command:
sudo rm /usr/lib/adobe-flashplugin/libpepflashplayer.so
If you're using Google Chrome/Chromium, it already comes with its own bundled PPAPI Flash plugin located at ~/.config/google-chrome/PepperFlash/some_version_number/libpepflashplayer.so. However, if videos aren't working with Google Chrome/Chromium when you're on Facebook, YouTube et cetera (you hear the audio, but doesn't see any video), then you're probably trying to watch HTML5 videos but your GPU (Graphics Processing Unit) has a built-in rendering blacklist that's avoiding video acceleration required by HTML5.
In order to fix this issue, start Google Chrome/Chromium, access this URL:
chrome://flags/#ignore-gpu-blacklist
...then activate the above option and restart Google Chrome/Chromium.
If the above solution doesn't solve the issue, follow the 6 steps provided at How to manually install Adobe's Flash PPAPI plugin for a WebKit browser such as Opera, then back up Google's PepperFlash plugin and create a symbolic link to Adobe's flash PPAPI plugin.
If e.g. your Chrome/Chromium browser is using PepperFlash plugin version 24.0.0.186 and you downloaded Adobe's flash PPAPI plugin version 24.0.0.186 (same version of Google's PepperFlash), then this is the shell command you'll have to issue after you install Adobe's Flash PPAPI plugin:
mv ~/.config/google-chrome/PepperFlash/24.0.0.186/libpepflashplayer.so ~/.config/google-chrome/PepperFlash/24.0.0.186/libpepflashplayer.so.bak ; sudo ln -sf /usr/lib/adobe-flashplugin/libpepflashplayer.so ~/.config/google-chrome/libpepflashplayer.so
Open a shell terminal window and run this command:
sudo rm ~/.config/google-chrome/libpepflashplayer.so ; mv ~/.config/google-chrome/PepperFlash/24.0.0.186/libpepflashplayer.so.bak ~/.config/google-chrome/PepperFlash/24.0.0.186/libpepflashplayer.so
Source: my own answer to question #470281.
