fix browser version validation code for firefox

This code kept asking to update the browser to Firefox 3+ even though I'm using the latest Firefox 25, because `SC.Platform.Firefox` returns "2". This is possibly related to this entry in [SproutCore's HISTORY file](https://github.com/sproutit/sproutcore/blob/master/HISTORY):
"- SC.Platform.Firefox now returns 2 for any gecko browser that is not Firefox." (v0.9.10)