I stopped using Firefox in favor of Chrome, years ago, when I saw Chrome had bookmark syncing and incognito Windows. Firefox has, since, tried to replicate these behaviors only to be way off the mark. When I want access to my bookmarks it's because I'm /not/ at my computer. Also, putting the entire browser into private mode, is not what I need, only a single window for when I'm browsing sites I'd rather not keep cache & history of. Now, Firefox continues to chase after Chrome like some weird kid brother.
Then there's this versioning issue. While there's no real standard to versions, there is sort of a convention that when the first number in the version changes it means you have either a complete rewrite of the source code or some fundamental change that makes the new version a very new piece of software compared to the old version. I refuse to believe that Firefox has been rewritten from the ground up 4 or 5 times this year.
At this point, I'll go back to IE before I'll start using Firefox again.