The best way to find out if Mozilla is better than Opera for you is to find out yourself.
I'll mention some things I like about Mozilla though.
<li> open source: what does this mean? Well, first off, it means that there are no ads in the free version. In fact, there is no non-free version. Also, it means that there is an infinite number of possible programmers. So, if someone wants something or something is needed, anyone can program it. That doesn't guarantee that it will be included because the core developers choose what gets put in, but it does help things happen a lot faster.
<li>
www.mozdev.org : a necessary visit for any Mozilla user in my opinion. Know that the things here are not guaranteed to work - there's a reason they're not in the "stable" version of Mozilla. That reason could be because they're incomplete or buggy, but it can also be simply that they don't see the need to make that extra feature part of what everyone has to take time downloading to use Mozilla. This is the place you can go to get an add-on for Mozilla to have mouse gestures like in Opera among other things. You can also find themes here. Not to mention lots of other useful (useless?) goodies. A similar place which should be visited is
www.xulplanet.com to get the PrefBar which I am particularly fond of.
<li> I repeat, no advertisements: That's right. None. NO advertisement banner like in opera, no AOL links like in Netscape, none. And freely.
<li> Pop-up blocking: This is in Opera as well. Nonetheless, it's a great feature. You can also tell it to allow pop-ups for specific sites, so I've told it to allow pop-ups for TFP this way when I get a PM I can have the pop-up window to read it.
I'm sure I missed a few too - perhaps meanspleen will have something to add since I know he is also an avid Mozilla user.