To be honest, it feels like you didn't actually look at the link.
2) Compatibility with Webkit-based mobile browsers for debugging. I'm so not having two browsers with hundreds of MB of RAM usage open.
Firefox' advantage is, though, the mouse inspector - it shows rulers for the selected element at it's borders. Invaluable when trying to align text across columns or rows!
2. Ummm...don't you have to test against non-WebKit browsers, anyway? Or, do you really expect all of your users to only use WebKit based browsers? That seems problematic.
Testing in Chrome on OS X takes care about Chrome, Windows, Safari, Android, iOS (even though the iOS Webkit has some quirks) and Firefox. Compatibility between browsers in rendering is not an issue these days any more, the only notorious exception being Internet Explorer, but even there the margin is getting smaller and smaller.
I usually develop and full-test in Chrome first, and do a quick "does everything look and behave correctly in other browsers" at the end, so far I have not encountered any major differences between the platforms.