Which Firefox extensions would you miss if you used Opera? Opera has built in support for tab thumbnails, mouse gestures, Greasemonkey scripts, widgets, etc.
https://addons.mozilla.org/en-US/firefox/addon/5890
Still not matched anywhere else. The bogging-down from ridiculous numbers of Firefox tabs may soon force me to switch, however.
I also like some of the highlight plugins for translation and search.
Opera 10.50 has new search highlighting, though, and the right-click context menu for highlighted text has included a translate option for a while now.