I tried Mozilla around 1 year ago and found it very unstable to the point of being unusable.
However after reading recommendations on the PCA forums I recently went back and tried version 1.4 (now 1.5) and the program is almost perfect. Some of the features that you get with Mozilla are:
Tabbed browsing Auto Form filling Password Manager Popup Blocker (this is the most impressive thing for me) Bookmark Manager is mutch better than IE
My only gripe with Mozilla is that very occasionaly it does still crash, but then IE falls over regularly aswell. One other slight problem is that a few sites don't seem to like it. Dabs.com is one site I still have to use IE to access properly.
I would recommend Mozilla as the positives massively outweigh the negatives. But as both Mozila and Opera are free, download them both and form your own opinion.
PS: I tried Opera, but Mozilla does everything I need so it was deleted, I'm sure someone else in the forum can fill you in on it's advantages.
Have had various versions of Opera - now using ver 7.21. Like tabbed interface, the wand that saves passwords and the skins (mine looks nothing like the default) Disadvantages are that it is much slower loading the program than IE6. Also, some multimedia doesn't work - the BBC radio sites for instance.
Pop up stopping was also an advantage, but since installing Google toolbar in IE6, this program is now free of pop ups.