While I am personally largely against extensions, I wouldn't be if their allocation was managed better. I think that the extension should say what websites it's intended for (Greasemonkey does this) and be in the javascript process for those pages only. It shouldn't be separate, it shouldn't be in memory before the page is even open.
I've been using Opera for years, if I did need anything I'd use UserJS (Greasemonkey done properly, built-in). I have no extensions, and only two userJSs (youtube download, and gawker fix). I have adblocker and firebug as built in functions of the browser, I have custom searches instead of things like Tineye extension that I have on Chrome.
