Here's some of the Firefox extensions I use that will probably be useful for power users (who are not web developers) like Alex. I'm not going to link to them because that would take me forever (at this level of tiredness), but they're all on the Firefox extension site.

IE View - This lets you easily view pages in Internet Explorer for those times when it's just not looking right in FF.

All-in-One Gestures - Provides gestures and a bunch of other important features. I think FF gestures are different from Opera gestures but you can easily change them. You can also add gestures for just about any action. I added LR for close tab (since it sorta looks like a C) and that's about the only one I use.

Tabbrowser Preferences - This lets you configure just about everything about tabs. I think by default FF acts like a multi-window browser and you have to mess with settings to turn it into a single window browser.

Focus Last Saved Tab (FLST) - When you close a tab, it normally focuses the tab right next to the one you close. With this extension, the last tab you looked at before closing the current tab will be focused instead. This is really a matter of how you expect tabs to behave. If you the default behavior is ok, then don't get this.

Tab Preview - When you hover the mouse over a tab, a preview of that page will show up.

Google Toolbar for Firefox - This is the official toolbar from Google. You might have to get it directly from them. Remember to turn off the pagerank display and form autofill (and possibly some more options) for more privacy.

Session Manager - This will save your sessions like Opera does. Plus you can manually save and restore sessions. It also allows you to restore closed windows and tabs.

Forecastfox Enhanced - Shows weather updates on the toolbar.

Menu Editor - Lets you edit the right click menu and other menus. Useful when you've got too many features taking up space on the right click menu.

That should be enough for you to customize. Most of my other extensions are for web development or for specific sites (Squidoo and del.icio.us) or for small features you probably won't care about.