Firefox does seem to be very efficient, but I love the mouse gestures and different skins for Opera. The mouse gestures I can barely browse without now. Granted I've used and researched Firefox less than Opera, so these features may even be available for Firefox(which they should be).
Also I use the session system a lot in Opera, it allows you to save and open different combinations of websites, but like I said earlier this feature should be there in Firefox(I personally just haven't researched it enough yet).
So, I guess all I can end up saying is I like Opera, but think Firefox has potential.