I just downloaded the latest Opera beta and did a test on both browsers. While Opera scores more than Firefox (78/100 compared to Firefox's 71/100) they both fail to pass the test. This may be a huge improvement over Opera 9.27 where the browser actually crashes half the time and gets 46 when it doesn't but its still barely better than Firefox.
I'm talking about the unstable builds of their renderers. Opera has released an unstable build on their renderer, which passes 100/100 on the Acid 3, and so has Webkit. Gecko still has not. Opera beta may sound like an unstable build, but it's actually a pretty thoroughly tested build.
Now that thats out of the way, what are the features found in Opera that aren't present in Firefox or an addon for Firefox?
First off, why would I want to download addons? Why should I need to spend hours of my time going to sites, downloading shady executables, just in order to use a basic browser? Addons do not count in a browser's features.
Custom panels and links on those panels is the biggest feature I use. I like to get rid of buttons that don't do anything, like home, and why do you need a stop button AND a refresh button at the same time..? I also like to link to all the sites I use commonly there, simply by dragging and dropping from the URL bar.
Quick search is also a big one. Right click any form, and click create search. Now, just by typing one word (or letter), you can run a search on any site, anywhere. I've got one set up for countless sites that I often use.
I don't know why, but in Firefox, tabs always open in the background, and there's no way to create a new tab without first going into the menus.
Speed Dial (gives you several links within a new tab) and Opera Link (allows you to use the same favourites and settings across multiple machines) are useful, ingenious features.
Opera did it with an internal build. I don't think they released it.
They did.
http://labs.opera.com/news/2008/03/28/
None.