That's the cool thing about Firefox - The Extensions system is VERY powerful and fairly easy to code for too!
With the right extensions, you can give Firefox all the features Opera has, e.g. mouse gesturing, internal torrent support, crash recovery, on-the-fly page hacking (as in Greasemonkey), improving things like the cookie, popup, download, search and password handling to levels at least as good as Operas (Seriously, do this; FF's baseline handling really sucks...) and more!
Heck, the only things you can't give it are Opera's speed and stability

I'd love to get a similar thing in Opera - You get a *damned* lot of functionality out the box, but that's pretty much it. You can customise what's there to obscene levels, but there is no real facility to extend what it can do as easily as the FF Extensions system (Sorry, but Widgits don't even come close... IMHO they're a horrible waste of code; They don't do anything that can't be done in a Panel...).
That said, I'm not sure I'd trade the Firefox Extensions system for Opera's speed and stability...
Erk... but have I started another flamewar?!

So... uh... how about that Lynx browser..? *runs*