Dude Firefox is slow after you use Opera. I'm saying this as a Firefox and Opera user. I use both but if I had to choose it would be Opera. Just give it a try(and i dont mean for 3 minz then uninstall)try it for a week or weekend, and if you go back to FireFox that fine too, every one got there own prefrence.
You're completely correct. On Windows, Firefox is much slower than Opera. If you have a few tabs open, there is a delay when opening/closing a tab during which Firefox seems to freeze. Opera doesn't have this problem; it seems the tabs open and close instantly no matter how many you've got open.
I use Firefox because it's open source, matches your chosen GUI on Linux, and is available in 64-bit. Opera, on the other hand, looks the same as it does on Windows so it looks out of place instead of matching your other applications. Opera isn't open source and there is no 64-bit version of Opera.