Yup, and I'm a little unusual because #1, I do website work for clients, and #2, I keep a crapload of things open all the time.
A long, long time ago I made FF my default browser because (at the time) it was the only one that could recover from a crash and reopen all windows/tabs that were open at the time of the crash. And it was the first one to have a session manager addon to save and reopen a group of windows/tabs without having to use Bookmarks/Favorites every time. These days, though, there are session managers for both IE and Chrome, too.
We have 8 systems always on here at the house, and I have 3 at my primary desk on a KVM. Always on. On each of those machines, I will always have a number of windows/tabs open in at least FF, IE, and Chrome. On the system I'm on right now, I have 11 FF windows open - one with 16 tabs, one with 4, one with 3, and the others just 1. I have 9 IE windows open, some with multiple tabs. And I have 6 Chrome windows open, some with multiple tabs. That's pretty standard, all the time. Part of the reason for the multiple browsers is that I'm logged into various sites as multiple people; some as myself, while at the same time as a client representative.
For instance, I can't be logged into a site like TripAdvisor as more than one person in the same browser on the same machine (cookie thing), so I use a different browser for each client I might be representing on the site. And when I run out of browsers on one machine, I have more on another (and another). And, I refresh them all regularly. Also, I have session managers installed with saved sessions for when the different browsers inevitably crash once a day or so.
Safari, Opera, and the lessers I generally just use for testing purposes. It's all a juggling act, but one I'm used to.
As many problems as there are with it, FF is my default on all machines and what you'd call my primary browser. IE would be second, just because it's a 'standard'. Chrome would be 3rd, with Safari and Opera somewhere close behind. All just a personal preference.
And there's the longer version of the answer...