IE 9 is out (and now 10 is being put forward for Windows
Opera 11.5 is there
Google Chrome is at version 14
Even Safari is at version 5 (I think - can't see any way to check for updates)
Firefox is still at version 3.5 which must be really bad ... oh no it isn't at version 3.5 any more. Someone farts in the office each week and that bumps the major version number up one!
I can't honestly say I have seen anything since version 3.5 that justifies a whole version increment - except the annoyance of constantly broken extensions. I haven't seen anything since version 4 that warrants any sort of version number change in the first or second digit.
Really extensions are the only reason I ever use Firefox - I am increasing using Chrome which is so much faster and just works.
Maybe I am being overly critical and there are major things going on behind the scenes that are building up to a major wow factor but somehow I doubt it. Mozilla is just turning itself into a joke with this ridiculous pace of version change.