OP:
I don't use Opera, but Gearbox makes it sound obsolete. :p It's a very fast, modern web browser that runs well on slower, low memory machines (like the Wii, cell phones, pdas, etc.). Firefox 3 has just managed to beat it in performance, but the Opera team is pretty dedicated and each new version is faster. Opera also helped popularize browser tabs, gestures and a number of other features. It's a cool browser, but I still stick with FF.
wfz:
The other Mozilla browser is most likely "Seamonkey" the old Mozilla suite browser.