I ditched Firefox for Camino three years ago and then upgraded to Chrome when I got a new MBP two months ago. I'll never switch back from Chrome.
Firefox on the Mac is brutally slow at times for me and the interface doesn't look nearly as nice as Chrome. I like software that gets outta my way and lets me work effectively. The developer tools in Chrome are way better than Firebug IMO anyway so no hangup for me there.
The switch from FF to Camino was because I wanted something based on FF but that behaved like a native OS X application. The switch from Camino to Chrome was mainly due to curiosity/hype/etc and it turned out to be true what people said about Chrome being pretty great. Another main consideration was the fact that I want to get into WebGL and Chrome supports it well.
Firefox on the Mac is brutally slow at times for me and the interface doesn't look nearly as nice as Chrome. I like software that gets outta my way and lets me work effectively. The developer tools in Chrome are way better than Firebug IMO anyway so no hangup for me there.