My little trial of Opera is over. I got fed up with its quirkiness & went back to Camino (at least until it starts crashing or slows down).
I too fluctuate between browsers. I haven't used Camino for a really long time, but maybe I'll try it again. (What's the practical difference between that and Firefox? They're both from the Mozilla Foundation, they're both Gecko, and they both have Cocoa UIs when run on OS X. Why hasn't Camino been rolled into the OS X version of Firefox? Just curious.)
I usually go back and forth between OmniWeb and Safari, and occasionally Firefox. Actually, sometimes I go days using only the built-in bare-bones WebKit browser in NetNewsWire, just for convenient access to my feeds.
Firefox uses XUL rather than a native UI and it doesn't use system facilities like the keychain (it has its own password storage), services, or Bonjour, and it doesn't support the middle mouse button. It is faster & more stable than Camino and supports extensions. I've also found at least one site that renders correctly in Firefox but not Camino.