What was wrong with the old Firefox versioning? Do they need just to follow someone else?
Anyway, no matter what Mozilla (or anyone else) does, we at Telerik should be ready to meet the news. The latest internal build of our Automated Testing Tools solves the compatibility issues with Firefox 5 so tests are now played back there just fine. We will support Firefox 5 officially with the 2011 R1 Service Pack 2 due in a couple of weeks.
Enjoy!
-Konstantin