The first thing a browser should do is display webpages. Until Opera 8.x I had struggled to make my webpages Opera friendly and finally I gave up. Opera 8.x seems to have an improved rendering engine, that takes into account many common quirks, but not as many as Firefox. We still live in an IE dominated world, so displaying webpages as similar as possible with IE is a must for any browser IMO.
I think Opera should rethink its priorities - instead of bragging with some incomplete SVG support that no one even uses they should focus on bringing their rendering engine on par with Gecko - after all a browser is all about displaying webpages.
On the XUL front ... it's nice from a technological point of view, but I think that native GUIs are much better - because of performance and seamless integration into the host OS. For example, Firefox menus look quite ugly on Win 2k and Win XP with classic theme. You have to install an extension (
https://addons.mozilla.org/extensions/moreinfo.php?id=1208&application=firefox) to make them look in place.
Still, Firefox's user interface is much more polished than Opera's, which might seem overwhelming for the average user. Another thing I dislike about Opera is the way the tabs look - they are more like buttons than tabs, and I'm also pissed by the fact that I can't figure how to put the tabs toolbar below the address bar like in Firefox.
Bottom line:
Firefox:
+ BETTER RENDERING AND JS SUPPORT (but Opera 9 shows promise)
+ more intuitive interface
+ all the features that an Opera user wouldn't even dare to think about via extensions
Opera
+ Faster and more lightweight
+ A few interesting, but seldom used features (zoom, style)
+ Better download manager
+ Built-in mail client
I think it's up to every user to make a choice depending on his/her needs.