I must admit that, having tried both Opera and Mozilla on a number of platforms, Mozilla remains my first choice.
No particular reasons though, perhaps the appearance. Or a feature (extra from XULdownloads) that lets you modify the user-agent specs as though you were a Mac PowerPC with IE5 or a Linux box with NS4 instead of say a Windows 2000 with Mozilla, but i reckon there's no real use in it anyway.
So I say it's just personal taste, as far as I am concerned. I like Mozilla because of its streamlined appearance, it fires up quickly (even in Windows XP) , has very customisable privacy settings and pop-up killer options and is absolutely free and well-maintained by competent people.
And it does boast the multiple tab feature
Just my 2p worth,
C