Firefox still remains my browser of choice: it has it's problems, yes, but for me, configurability is king. (Chrome, what do you mean I can't - even with an extension - mousewheel to change tabs or set up a mouse gesture to minimise the window?) And IE is a total non-starter on that front: it's rubbish defaults are pretty much all you are going to get. Things like Adblock (and now, Noscript) are essentials as far as I'm concerned, and (since Opera, see below), so is the ability to configure tabs and set up mouse gestures - it's just so much faster having proper control and the ability to configure things. This is also why Safari will never be an option: a while back, when we had a weird proxy issue, Safari was the only browser that didn't let you into the proxy config enough to fix it - the stock answer for Mac users then was "You'll have to use Firefox".
Over time, it has been something like: Netscape (for the tiny bit of my internet-accessing days where that was relevant), then Internet Explorer (because there was no choice when Netscape imploded), then Opera (this was a good decade ahead of the others in terms of features for quite some time), then Firefox (when I finally gave up on having to keep switching to IE for all those sites that didn't support Opera - at the time, Firefox was more supported in more places). And Firefox it has stayed, for a long time. Chrome has never sat well with me: too much memory hogging, not enough ability to configure it (and not as good on the extensions front); minimalism is fine for some, but I want to be able to put exactly what I want, where I want (and yes, this means I have no less than 6 buttons on my toolbar for extensions - and I use all of them).
Also, Firefox is one of the only browsers that still separates the search and URL bars. If I want to search, I will use the search bar - I do not want you trying to hit up Google Search just because the slightly unusual URL I typed doesn't look like a URL to you.
Having said this, browser use in recent times has become more heterogeneous. There are times at work that I can't avoid IE (e.g. intranet; also, specifying that IE is the only browser we should be using is an utterly retarded decision on the part of our IT department - but thankfully Portable Apps exist). Chrome tends to get used for times when we don't want to reload a session of 10+ tabs for one thing, or for video streaming, when we don't need to be multitasking. At times, it's now "whichever browser is closest", although Firefox to me is still the best, as it's one of the few that actually still lets me make decisions for myself on how the browser should behave.
This idea of Chrome-only apps that's starting to emerge is horrid. Please do not do that. (I have an Android app that I would use on the desktop as well, if it had a version for anything other than a Chrome version - it's not worth another browser just for that).
Another reason I want to keep using Firefox is that it keeps a third rendering engine in the game (although I am getting concerned that this is starting to be lost): Firefox was a hero back when it finally managed to eke out just enough market share that "Designed for Internet Explorer" ceased to be valid, and we finally saw innovation return to the browser scene (arguably, this paved the way for things like Chrome to exist). The three-way Trident/Webkit/Gecko* scene we have had has seen more browser innovation than ever, but I worry that if we drop back to only two, we might see a duopoly that stifles innovation. Oddly enough, Microsoft's decision to revamp but not go Webkit was actually worth applause I think (not that I'm going to use it) - they apparently did this because they wanted to avoid a single-browser-engine world (though the irony of that shouldn't be lost on the audience here).
*Sorry Opera, you were great, but never quite big enough to make the difference you deserved; RIP Presto.