Opera 11 has replaced Firefox as my default browser. Here's why:

• The currently-selected website in Opera 11's hierarchical bookmark list stays visibly selected, even when the bookmark list loses focus. Firefox stops displaying the selection when the bookmark list loses focus. Bad!
• Opera 11 is able to suppress annoying Flash-based advertisements (actually Firefox with Flashblock does a superior job).
• Opera 11 is much faster than Firefox. I have always wondered why Firefox refuses to cache some graphics on websites that I visit on a daily basis.

I will still keep Firefox around, because some of the extensions for website development are must-haves: Web Developer, Firebug, and Live Http Headers.

Sadly, Opera 11 cannot stream Netflix movies yet. Hopefully this will be fixed in the near future.

Postscript: As of December 2011 I've switched back to Firefox. I got tired of Opera not rendering some websites correctly, plus I missed the wonderful Flashblock add-on.