Thanks for the article, Robin! It seems more like a general “why Opera is good” article instead of a “why Opera is better than Mozilla” one, but it is still really good.
Mozilla does have IRC software (Chatzilla), but it’s really not user-friendly in my opinion. Mozilla has a pop-up blocker as well.
I could have sworn that you could load bookmarks in the sidebar easily, but I couldn’t find it! I guess I am mixing up Mozilla with Firefox.

As far as security goes, you can compare the Secunia reports of both
Mozilla and
Opera (lastest versions). Both are really good. In the past, Firefox and Mozilla users might have been presented with a naughty extension that could have done some damage if accepted, but since extensions can now only be installed from “trusted” sites by default, I doubt people will see those.

Opera has better CSS2.1 support than Mozilla.