Firefox doesn't crash and pretty much works everywhere, and Safari doesn't crash (although it only works on Darwin); that doesn't sound like a differentiator.
> Firefox doesn't crash and pretty much works everywhere
Hell, it works more "everywhere" than Chromium (let alone Chrome) does.
Not in my experience. Developers develop for chromium, I find many more rendering issues with Firefox's rendering engine than I do Chrome's.
I meant more in the sense of operating system + hardware combinations on which you can compile/install/run Firefox.