It took me a while to find a good source of old versions of Firefox. The need arose while trying to reproduce a bug that the client reported only happened using Mac Firefox 3.0.1 (several versions back).

Most of the places that link to Firefox downloads update to point to the most recent release. A few that did have old version were windows only, and I needed the Mac release! Even Mozilla's website, usually the reliable source only had links to 2.0.0.5

So, back to the old FTP Site.

ftp://ftp.mozilla.org/pub/firefox/releases/

It has everything! All operating systems, all versions back to 0.10 it seems, and all languages! Should make future QA much simpler.