I noticed that Firefox 21 doesn't work in Ubuntu 12.04 (I upgraded Ubuntu recently). It pops up a message which says: "Your Firefox profile cannot be loaded. It may be missing or inaccessible". I was looking for a solution and nothing worked for me. So I downloaded Firefox 20.0.1 (the .tar file), I installed it and it works, but I don't know how to install it properly. I mean, when I install Firefox from the terminal or Ubuntu Software Centre it appears in the panel with its icon and all that. But installing it from the .tar file is just paste the extracted folder in some direction (in my case /opt/firefox/firefox) and to run Firefox I need to go to that direction and clicking in the firefox file. What I really want to know is how can I install Firefox so it seems to be as installed from terminal or Ubuntu Software Centre, I mean, with its appropriate icon in the panel, and with the possibility to running it from terminal using the "firefox" command. Or if necessary, in which system folder should I paste the extracted folder?
I think you're better off figuring out why Firefox 21 isn't working for you. The problem might not be with Firefox 21, but with a corrupted profile that will affect all Ubuntu versions of Firefox.
That aside, you can load Firefox 20 from the lucid-security repo:
On my system running 12.04 (precise), part of the resulting output shows: