Since I didn't use Opera as my main browser, I felt a little guilty. It's hard to express why I chose to use Firefox, but someone just did that and wrote an interesting article:

"Nowadays Opera seems to be grumpy and wanting attention for its past innovations and the things it is trying to do to keep up with Firefox and IE7. (...) In version 9.0 of Opera they released widgets to try and compete with Firefox's extensions. They seemed to entirely miss the point of how Firefox's extensions change your entire experience, and in turn created something less than thrilling. (...)

Web designers tend to try and support what the majority of their users will be using. They want everyone to use their web-apps, but tweaking for each browser takes time. Opera is a Web 1.0 browser, not because of Opera's own doing, but because the majority of Web 2.0 does not have the time to make it compatible."

So Opera can't escape the underdog complex, while having an elitist attitude.

"IE 7 comes out and adds tabbed browsing, but Opera has had that for 10 years. Credit is due Opera, and we'd like to see that reflected in market share," says Hakon Wium Lie, Opera's CTO. The problem is that people don't really care about who had this idea (Opera didn't actually invented tabbed browsing), and they don't care that Microsoft has said for many years that tabbed browsing is useless.

If Opera adds a plug-in system similar to the one from Firefox, improves they way it handles feeds, solve the JavaScript inconsistencies, fixes its Rich Text Editor, and has a more user-friendly interface, people might have a hard time choosing between Firefox and Opera.

The extensibility and simplicity is what makes Firefox great and Opera should learn from that.

Related:
10 great features from Opera

Google sites that don't work in Opera:
Google Calendar, Google Pages, Google Docs, Google Notebook, Picasa Web Albums (partially). </img> </img> </img> </img>