well, firefox is supposed to be "testing" is it not? regardless, since phoenix 0.1 i have found it to be very stable, although it *definitely* depends on the build you get. some windows binaries were quite horrible around 0.5-0.6 (and then i stopped using windows for the 'net so i don't know) and i have never yet had a good rpm (third party of course) of firefox, but the mozilla.org (static i think) binaries are rock solid.
also, i find they use netscape type plugins 100% ok, i mean i only use the java, helix player, and flash player plugins, and they've always been dead right. oh yes and plugger too, which is very good at piping things through other stuff within the browser window (not that it gets used a lot).
i did used to advocate opera since it's better than netscape, and was better than mozilla (featurewise) when i was using it, but now firefox + prefbar really *almost* contains all the features i ever used with opera anyway, plus it's free and open source (no ad banner too)