Opera is a non-free, very good web browser. FireFox is free web browser that so far I'm undecided on, but I'm giving it a good try. Having been spoiled by Opera, I spent a fair bit of time re-creating some of Opera's features on Firefox, hence writing a page on migration from Opera to Firefox.
It turns out that while the base Firefox is missing many of Opera's features, most of them can be added by using various plugins, which I'm listing (and keeping copies of) here. I make no guarantees that these plugins are up-to-date! If any are out of date, please let me know.
If you're clicking on the plugin from Firefox, which is very much what you want to be doing, a bar will pop up saying "To protect your computer, Firefox prevented blah blah blah". Click the "Edit Options" button at the right and then click Allow and OK. Unless you don't trust me, of course. :-)
As the things I like about Opera are mostly focused around the usage of tabs, that's what this page concentrates on. If you have a favorite Opera feature that isn't handled by this list, please please let me know.
In order of importance.