Opera is not bad. Neither is Safari. However I'll stick with Firefox.
Why? because I can customise FF using add-ons in ways that neither Opera or Safari let me.
I prefer to block ads, Flash, selectively, restrict Google's ability to track me amongst other things. I've yet to come across a browser that lets me do this as easily and as well as Firefox.
I know that people use PithHelmet with Safari. I know that Opera has a blocking feature. However neither is as good, (and in the case of Opera, as easy to use) as Ad-block.
Yes, I could use Privoxy, but why run yet another separate App to do the job of an add-on?
As regards the abomination known as Flash, I've yet to come across anything better than Flashblock.
A minor point. I quite enjoy Stumbleupon. Neither of those Browsers support it.
I could mention other things but I'm sure that other TS users will do that for me
Opera "can" be customised, but hey, do you really want to spend all that time doing it. It's like using KDE on a Mac. You "can" do almost anything if you want to spend the time and energy doing it.
Personally I'll do it the FF way. Use accredited Add-ons
Sandy