The 'suite' aspect of Opera is really unnoticeable if you're not looking for it. Integrated bittorrent irc + m2 Mail client are nice but really inobtrusive. It's not like Opera is a behemoth download
I'm no Opera fanboy, in fact I switched to Firefox from Opera a couple of years back, but primarily for the extensions - pederick's webdeveloper / adblock / bugmenot are indispensable. Since then, most extensions I have installed are to emulate some of the Opera features - stop&reload / gestures / paste+go.
What Firefox is not even close to Opera on is speed, especially relative to memory footprint. The 'quick' back+forward that is enabled with the 'not-a-memory-leak' cache in FF is a really poor second to Opera's handling.
And the feature I see wanted most by ex-Opera users is the full-page zoom. Implemented so it is incredibly quick, and totally reliable. The new IE7 zoom, and any of the Zooming extensions for FF are really pale imitations of this.
Add to this the tab flexibility gained from a proper MDI, the free-as-in-beer for desktop, and countless other minor features, all they really need is one more and I'd be straight back there...
Compatibility with Firefox Extensions