Been using Opera since 2006, when Firefox was starting to royally piss me off (it started getting bloated, and would randomly delete half my bookmarks on Linux and Windows).
This is my ideal layout in Opera. The tabs on the left (hidden by F4), make Opera ultimate in my opinion. You can even drag multiple tags between windows (useful for multi-monitor). You can hide the File bar with Alt+F11 in Linux (Windows too if you enable the hotkey).
When I have to use Firefox, vimperator makes it at least bearable.
Still, I don't understand. Opera has a mail client, IRC client, web server (Unite, dunno if its out of beta yet), and Bittorrent client, but up firefox takes longer to start up (standard install).
I had the chance to write a patch for Firefox in Linux a while back (for work). Damn, the layout of the configuration files made my head spin. It was almost as bad as looking through the OpenOffice source...
Does Opera have spell check built in?
Latest version does.