Excuse me for resurrecting a dead thread, but this is an interesting subject.
I was a long user of Firefox until I discovered that there was an alternative to the big four that wasn't dead or buggy as hell. Opera is my browser of choice for multiple reasons:
-It has the best speed dial out of any other browser
-Mouse gestures make my web browsing faster and more interesting (I don't even need the forward, back, or refresh buttons anymore)
-It's almost completely customizable despite it being closed source (and sadly, we'll never see an open source version of Opera), less so than add-on king Firefox and much more than the locked down application that is Google Chrome
-It has TAB stacking! TAB STACKING!!! I'm a big tab user, especially when I'm researching, and being able to group multiple tabs according to subject or task is really, really helpful. As far as I know, tab stacking is Opera-exclusive at this point. If there's an extension for it on Firefox (and there's an extension for EVERYTHING on Firefox), I don't know of it.
-It has a built-in BitTorrent client, although I admit that I've never really tried it. I always use uTorrent, but the idea of actually having your browser handle a torrent download from clicking a torrent link you probably found using your browser only makes COMPLETE SENSE.
-My absolute favorite feature of Opera, the Opera website has a download for its on built-in mail-client/RSS Feed tracker. For those of you who have never tried either, that means I can check all of my emails (3 accounts is hard to manage) and keep up to date with the latest headlines from all of my favorite news sites all by clicking a little panel in my browser. I don't have to enter a single username or password, scroll through big, clunky webpages, or type in a single URL.
-I'm not going to ever use it because I'm not into web development, but Opera also comes with a great web development tool called Opera Dragonfly that is indeed built into the system.
Firefox is still more powerful than Opera and it has a lot more extensions and add-ons than Opera does because it has a huge market share, but for my purposes, I really do prefer Opera.
That said, I actually tried out Maxthon today because today is the first I've ever heard of it! Not from this thread, but from whispers across the web. Really coincidental. My analysis? It's fast. Even faster than Chrome. It has a lot of neat features, my favorite being a snapshot tool that lets you take screenshots of whatever size region you want and directly saving it to your hard-drive or copying it to your clipboard without you having to go into Paint and croppin' and stuff. Opening up Paint was never a hassle for me but that tool really is neat-o. It also comes with its own RSS Feed organizer (which is really, really neato). There are probably many more neat things about it, but I didn't look for them. I won't use it simply because it has no mail client, and because the UI is... well... it's ugly. It's really, really ugly. I downloaded some skins, but it's just... BLEH. Uninstalled from my computer. ...But I must say this again: it's lightning fast.