What versions do you test with when testing for Firefox compatibility? Firefox 3 is a no-brainer, but do you then test for 4, 5, 6, 7, and 8?
Do the latest version, and a quick check with the previous if you really want. There's a post someone did a few months back with graphs showing what happens to various browsers' versions once a new one comes out. It was primarily about Chrome, because of its automatic updating, but he also saw that since 4 Firefox has been doing basically the same thing: as soon as a new version is released, the previous one basically disappears and is no longer a consideration. (Once you factor in the people that are stuck on 3.6, which you're already aware of.)