I've been using Opera 9 as my default browser for the last 6 months and here are some of the features I'd have a hard time living without if I moved back to Firefox.
Detaching Tabs and True MDI
Quite often, I'll want to compare the contents of two web pages side-by-side. The problem usually is that the web pages in question are usually already loaded as two tabs in the same window. With Opera, I can drag that tab out of the Opera window and it becomes a new stand-alone Opera window. I think this is a feature that even novice web browser users could appreciate - think of comparing prices/features on two different items in Amazon or on different websites.
Opera also supports a true multiple-document interface that allows you to cascade, tile, minimize and restore tabs within one Opera window. I've missed true MDI ever since the world decided to steer clear of it years back in favour of a tabbed-based approach. Mozilla has stated they will not support MDI in Firefox either. Every so often I will use this feature in Opera and I really appreciate it, but it's hidden enough from the novice user that it doesn't overtly confuse them. I think Mozilla should take a page from Opera on this one.
The Wand
At first, this Opera feature actually annoyed me because I was used to Firefox' ability to remember the passwords, but I've found that Opera has done a better job for me in remembering passwords over the long run. Now I just go to a web page and click the Wand button (or ctrl+enter) and I'm logged in automatically, no more searching for the Submit/Login/Go/whatever button on the web page.
Tab Trash
Sometimes I'll close a tab that I didn't mean to. Opera has this great little trash can button on the tab strip that lists the most recently closed tabs and I can select the one I want to restore. I should mention that Firefox 2 recently added the ability to "Undo Close Tab" (right-click on the tab-strip), but I don't think that's quite the same thing.
Bleeding Edge Standards Support
Opera is much further along in supporting SVG [SVG11] than Firefox is, especially in terms of declarative animation. I've been a fan of SVG for over a year now.
Opera is the first browser that I know of to support the proposed Sound interface of HTML5. I've found one good use for this feature in a web application I've been using/developing for a year and a half. Web games are another obvious target.
On the other hand, I still love Firefox for:
Extensions
Combined with the thriving Mozilla fan-base/community, extensions become the number one killer feature to have for a modern web browser. For instance, I want to be able to see my Adsense revenue in the status bar, or right-click to automatically bring up a browser tab in Google Maps. These type of things are impossible to achieve without extensions. C'mon Opera!!
DOM Inspector and Other Developer Tools
The Mozilla DOM Inspector and now the Firebug one. Being able to trace XMLHttpRequest traffic. I know Opera is in the works to deliver some developer tools in the future, but this was needed two years ago! C'mon Opera!!
And here's a feature that browsers don't yet have that I'd like to one day see:
Split View
Have you ever wanted to compare two sections of a long web page at the same time? Text editors, word processors, spreadsheets have the ability in the scrollbar to grab a little widget and create two views of the same document. Why don't web browsers allow this?
Mozilla seems set against doing it, but if you like the idea go vote for Bug 231156. Maybe someone will re-open it?
I filed an RFE about MDI (without knowing the acronym) for Mozilla once. Thanks for telling me Opera does it (and Mozilla is not even planning on it). Mozilla should do xml:id, MDI and SVG 1.2 and give its SVG developers some extra hand$. There are a couple of Firefox extensions i can’t do without. I promote both browsers.
Re: Split view,
Konqueror has had split panes for a long time (not quite the same … but). Load a page and copy its address. Window – split view. click in the empty pane, and paste the address into the location field.
Opera can do something similar using its MDI.
First time only, you need to enable MDI;
Tools – prefs – advanced – tabs – untick ‘show close button on each tab’
Tools – prefs – advanced – browsing – show window menu
Window – minimise all (if the page is the only tab you have open, you can skip this step).
New tab & open the page you want (or just focus the tab if it was already loaded in a tab).
Window – duplicate.
You should now have two tabs that are not minimised – any others that are open will be minimised (although you probably can’t see that).
Window – tile vertically (only the tabs that are not minimised will tile).
Ok, it’s not really the same as split view because any changes in one pane are not visible in the other, but it works for many purposes.
Thanks Tarquin. Yes, I’m aware you can do something close to Split View in Opera using MDI, and it’s good to know that Konq does it too. But it’s hardly as easy/intuitive as simply dragging that little widget down in Word/Excel.
Interesting thing about MDI support. Ignore Mac and Windows for second … Mozilla (FF, etc) is built with GTK in Linux. Currently GTK itself does not support MDI (stupid, I know). So I would think that this alone would cause a great deal of trouble. Even if they wanted to support it, they would have problems on Linux.