At work we have a web application purchased about 18 months ago that officially don't support Firefox above 3.6. There were rendering differences between 3.6 and 4 that were serious enough to enforce that restriction for certain users. Apparently, it has begun working as expected again in FF7 (might have worked in FF6, not sure).

The application also uses jQuery 1.2.6 (released May '08) - I guess I'll be the one to QA the migration to the present day.

Hence I am not yet a believer in "latest version is the greatest version", in all cases. In principle, yes, but there are exceptions that makes version numbers necessary.