Many times, I will go to right click on something in Firefox 4, and the context menu will disappear the second I move my mouse. I can use the arrow keys to select.
This comes and goes. When I was running Maverick, and switched to Minefield, and it began to happen, I wondered if it was something to do with all my customisations and Compiz, but now it is happening in my fairly stock Natty amd64 install.
This seems to be a bug with Unity; I've looked for a report on Launchpad but the keywords are so generic that it's not an easy search! Anyway, for me the problem occurs as follows: 1. Open Firefox on Workspace A. 2. Do some other activity (maybe email in Thunderbird) on Workspace B. 3. Use the keyboard to navigate back to Workspace A. (i.e. Ctrl+Alt+Arrow) 4. Right-click and URL drop-down menus will now fail to work properly.
However, if I move back to Firefox by clicking on the Firefox icon in the Launcher, things work just fine. If encounter the problem I just go back to Workspace B, and then use the mouse to return to Firefox. This is a bit annoying though, as I prefer to use the keyboard as my primary input.
I have encountered this problem in Firefox 4 as well, and unfortunately I have not found a permanent solution, but minimizing the window and then restoring it seems to fix the problem for me, so I'm guessing it should work for you too (hope it does).