I have compiled a list of older Firefox versions on this Firefox Review web page. The links are from the official Firefox website for Windows (US-English version) - versions 3.6.24 to 7.0.1. However, please note that Firefox recommends using the latest stable release version because earlier versions may have security and safety vulnerabilities.

That listing is nice, but it links to Win32 download folders. Not much use for a Firefox user on a Mac OS.

Mac users need to click twice on Parent Directory to get to the folder level where they can enter the Mac/ folders.

http://releases.mozilla.org/pub/mozilla.org/firefox/releases/7.0.1/mac/en-US/

Actually, it's been updated with Mac versions as well.

Wow, that was fast!