I haven't actually looked at it, but I have the feeling that the Firefox code base is godawful and nobody who isn't being paid for a 40 hour weekday actually wants to get involved in it.
Damn, didn't know that one can develop skills to evaluate the quality of a code-base by just "feeling" it and not looking at it at all - quite impressive!
Either outright commits by at-google email addresses, or things like summer of code. All to play catch up with the features they shove on chrome.