On the Mac, Safari is far more power efficient. With each update I notice Firefox is using more and more ram too. Also good luck finding out which tab is causing problems.

Last time I did a DOM benchmark Chrome still beat it by a significant margin. Also, only personal experience, but Chrome seems way more responsive with multiple tabs open than Firefox.

One annoying thing is that current versions of Firefox seem to rely on disk I/O too much, so if it's busy, you can't really do anything (switching or creating new tabs is greeted by spinning loading circle that takes a long time to happen).