IMHO rendering is not the problem of Firefox. The problem is that it interacts with the OS very slow.

One example: Try to listen to music via bluetooth stereo headset, reading mails and jump often back to Firefox... it freezes very often and the whole system becomes unusable.

I do like the Firefox UI concept and also the plugins but at the moment I can't use this browser as default one. It's getting better from version to version but is far away from it's goal to be a real alternative browser.