It seems apparently easy:
sudo apt-get install firefox-2
Now, I can run Firefox 2 from the command line with
Firefox 3 still works, by running 'firefox' - so while I can use 3 for browsing, I can use 2 for developing.
Note, I had to create a new profile for FireFox 2, because it didn't seem right to run it with the version 3 profiles (and the extensions had a few issues) - but this is fair enough. See Firefox Profiles for more information.