That's nonsense. Opera is based in Norway. I believe that the Mozilla Foundation spawned from the wreckage of Netscape, funded by AOL.
As far as I know it uses a similar tab system, that's all I know.
No, Opera has true MDI functionality. I imagine you understand what I mean there... you're a programmer, aren't ya? For the ignorant, it means windows-in-windows, as if you're in Word with multiple documents and go Window -> Arrange All, or Window -> Tile, Cascade, etc. Much,
much more useful. Can do some superb things with vertical tiling and window-linking...
I really can't bear to use Firefox anymore. I like the mentality of open-source stuff, the freedom, etc., but, well, Firefox just isn't as amazingly comfortable to patter around the Web in, and it's hardly going to be a tightly-coded speedy little beasty when you have to get community-written plugins for any kind of useful feature (often copied from Opera [really]).
For the record, I do use Thunderbird. It's a nice program, but it's not Opera-level stuff, and all Mozilla appies feel slighty slow to me.