I've been using Firefox since it was called Firebird, and before that, I used the Mozilla Suite that Phoenix/Firebird was birthed from.. However, I found myself using Chrome a few years back and haven't made an effort to stick with Firefox for my main browsing application. This was the best type of usurping as I see it: It wasn't some hard comparison that had me splitting hairs over which browser suited me better, they both did the job well; I just naturally started using Chrome more and more til eventually it became my main browser. Like some unconscious bias toward Chrome for simply annoying me slightly less than the others... I still use Firefox for the firebug extension when I need to check some web work... And of course, I wouldn't use IE if I was forced to at gun point. So FF over IE any day. But I too have moved on from FF as my main browser.
I couldn't make a clear cut distinction between FF and Chrome if I had to... just recently I started using FF, it's more open sourced and maybe less compromised by spy agencies? I'm having more fun with the add-ons? The color scheme even... Google's stuff is really good, no doubt about that. I was thinking that the Google Docs editor was sooooo primitive and how much better Word would be.. until I started making the switch and decided I like the Google editor better for a few reasons. I just take notes and make plans in Google docs, it's not like I'm writing a dissertation or anything... Little annoyances that I have no control over are what gets me looking over the fence for something better. In the case of the docs it was the lack of a quick insert command for date-time.. in the case of the Chrome browser it was the annoying tooltips...