I tried to install FF3 a while back, but the default install from Ubuntu replaced FF2, which was irritating because some of the plugins that I use on a regular basis don't yet work with FF3. So here is how to install Firefox 3 while keeping Firefox 2 and all the nice plugins you have for FF2 running at the same time:
1. I assume you don't have FF3 even downloaded yet.
2. Open a terminal window.
3. Start FF2 with profile manager:
> firefox-2 -profilemanager
4. Make a note of the name of your default profile, it's probably named "default".
5. Create a new profile named FireFox3.
6. Uncheck the "Don't ask at startup" box, then start Firefox, then exit.
7. Edit your start up link for FF2. I'm using KDE, so I right-click on the icon and choose "Configure Firefox 2 button". In the "Command line arguments" box, enter "-no-remote -P default". Change "default" to the name of your default profile if it is different.
8. Go to www.mozilla.com and download FF3. Don't use a package manager, get the .bz2 file.
9. Unzip the downloaded file to wherever you want Firefox 3 to be installed. There is no installer, just unzip the file. I put mine in ~/apps/firefox-3.
10. Create a new link to FF3. In KDE, I right click on my panel, choose Panel Menu -> Add Application -> Add non-KDE application, then fill in the blanks. For the Executable, pick "firefox" from the directory you unzipped your download file into. In the "Command line arguments" box, enter "-no-remote -P FireFox3". Set the icon if you'd like.
That's it. Now FF2 will run as expected with all your plugins, and FF3 will run and be ugly, but fast.