I had the same problem (issue with Google Apps and XUL runner etc) of needing to downgrade. To pull it off I went to the main Firefox download page (http://www.mozilla.com/en-US/firefox/new/) and followed the link to OTHER SYSTEMS AND LANGUAGES which took me to a page that included a link on the right hand side to download Firefox 3.6.16.

(The stuff below pertains to Windows. If you are on a different OS your steps will vary) To do the actual downgrade (and there may be some other steps that would be wise to take but this is what worked for me) I merely renamed "c:\Program Files\Mozilla Firefox" to "c:\Program Files\Mozilla Firefox bad" (in case I want to try troubleshooting the issue later) and then ran the "Firefox Setup 3.6.16.exe" file that downloaded from that link. That solved my problem and I'm back and running 3.6.16.

In case you don't feel comfortable changing things in your Program Files directory you might try a different approach and try uninstalling via the Control Panel and Add and Remove Programs (or whatever it is called on the version of Windows you are on) and then running the installation file. I didn't go that route so I can't tell you how well that might work.

Thank you so much dkp. After I posted this question, I found the old file on my computer. However, my computer is going to the "hospital" for a week and it looks like the hard crash is about to crash for the second time (only after one year). So, I am so grateful for this response as I will probably have to use it once it is back.