Opera - for the simple reason that I've been with it for so long now [since way back when Netscape Navigator had the majority of the browser market]; and 'cause I actually use the mail client, too - no interest in replacing that one program I like with two different ones.

Opera, and have been since 1998 version 3.5. Loved the cleanliness of its design and quick, no-fuss loading of pages. That's remained its biggest selling point for me years on. The Norwegian provenance also gives it some nice cred.

I loathe MIE. I find Firefox to be a bit too bloated; plus by the time it really caught on I was already an Opera devotee. I like Chrome as a back-up for loading pages that are Opera-unfriendly, however. At first I didn't see the point of its introduction in the "browser wars" but Chrome is a very servicable browser.

I prefer Opera as my main browser too.

Occasionally I use Chrome, because some pages don't work as expected in Opera. I couldn't checkout on GOG.com for instance. I don't know if that issue has been fixed, but I still view GOG's page in Chrome.

I prefer Opera, personally. I like it's overall set up. It works quickly and cleanly. After that, I'd probably say Chromium (I use it as a back up browser on Linux). It's fast as hell, but starts to lag a bit too much for me with too many tabs open. On top of that, a lot of pages don't seem to render correctly, as if the browser is taking shortcuts to improve speed. I also wish adblocking was a bit better. After Chromium, Firefox. I started having issues with it, though, and I never used many plugins that made its use necessary. On top of that, it's slower than both Chromium and Opera (for me), so I don't see much of a point in using it. Finally, IE9. Pretty decent compared to previous iterations of IE, but it's still the lowest on my list. That said, I do use it as my back up under Windows Seven.

Firefox

Chrome for casual surfing.

Firefox when I want to do anything important, need to run specific plugins, or do debugging (with Firebug).

IE, Safari, or Opera only when forced to.

Firefox, with mouse gestures add-on and with speed dial. =)

Firefox. I prefer the ever-important Adblock Plus, Flagfox, and Speed Dial addons on Ubuntu.

On Windows, I bolster defenses with NoScript in addition to the ones I've already mentioned. I also use that Personas add-on.

On both OSes, I also added an add-on to show the IP address of the site I'm on, and one for my IP address. I also recommend Read Me Later.

Tried that add-on some time but found out that it really can be a handful.. In every site you had to accept something or many things. It wasn't for me.

No need for NoScript, Firefox with just Adblock Plus is fine.

Not true! Adblock doesn't protect from cross-site scripting. (XSS)
If a site is hit by XSS, you'll be redirected to a fake site, or the info you were sending gets snapped up. NoScript protects you from this.

And yes, it can be a handful, but it's easier to use now. Trust me, I've been using it for a while, and even got my parents using it.

Disable third part cookies?
Apps? You mean extensions?

Whatever

That wouldn't do it. XSS uses JavaScript.