Intuitive tab navigation? Not sure what you mean, but try getting the MiniT extension. Also try setting your General preferences to always show the tab bar.
For mouse gestures, get the All-In-One Gestures extension. I've found that even more customizable and useful than Opera's mouse gestures.
Spell Check in form fields, sorry, don't think there's a way to do that in FireFox.
Your Opera passwords might be able to be imported into FireFox. Not sure, but I know some Opera settings can be imported.
FireFox has very customizable toolbar placement, particularly if you get the Toolbar Enhancements extension. This one even allows you to move the buttons to either side of the tab bar or the status bar.
Opera-style customizable scrollbars are kind of possible. You can drag and drop the buttons wherever you want them to be, but you first have to right-click and choose "Customize" to get the customize window.
Simple sidebar switching is possible via the Sidebar Orientation extension. A restart is required after making adjustments, but I think things like that are going to be polished a bit before 1.0.
Page source editor choice is possible with the Mozex extension.