It seems the previous update was causing problems with the Facebook game, users around the world complaining they were no longer able to play Farmville because a bug “hangs the browser long enough for [a]timeout to trigger and kill it”.
The current version gets around this by increasing the amount of time required to “kill” an application while Mozilla work on a more lasting solution.
So the world’s second most popular internet browser is updated just to fix a single game. If for some reason you hadn’t realised just how big this game is until now, this should do it.
FarmVille Bug Helps Prompt Firefox Update [Gamasutra]