It seems the ppa for Firefox Developer Edition (formally aurora) is no longer active. I have installed Firefox Developer Edition from Firefox's website and installed it in /opt/firefox. Now it is asking me to update and I get a new tar file. Do I have to overwrite that directory every time I want to do an update? Of note, related post:
do not seem to work for me.
Did you open the PPAs in a browser and navigate upwards? That's all I did for this answer (starting with a link from your provided link that gave me a forbidden error)
You'll eventually see the PPA being used is
https://launchpad.net/~ubuntu-mozilla-daily/+archive/ubuntu/ppa
This may not be all they provide - I just followed links to an alternative.
Do note however - I have no experience using this 3rd party source; having never used it, thus cannot say this is what you're looking is suitable, fit for purpose, nor what you're actually looking for (even your unstated architecture).
You just need to change the permissions of your install folder in /opt as such:
sudo chown -R $(whoami): /opt/firefox/
