I know exactly why I originally switched: I was not yet educated on the sheer depth of Google/Chrome's hostility towards your privacy, it had a huge selling point: silent, background updates where I don't even know they're happening (I like it that way).
This was at that time shortly after Firefox went to that crazy new release schedule and whizzed along from version 4 to version 26 in what felt like about 20 minutes. It was during this time that Firefox was driving me ABSOLUTELY NUTS popping up every few weeks with yet another new release, and getting in the way with its addon checks etc next time it's launched.
There is little that infuriates me more than when I launch an application and can't use it for the next 30 seconds because it's picked that time to chug around doing updates. I don't know if it's because they're too busy coding to use their own applications, but very few developers in the world seem to understand that "when an application is launched, that's when the user wants to USE that application" (sometimes urgently), and therefore very few have the sense to do their updates in the background or on exit (still annoying if you're waiting to shut down, but at least slightly less annoying).
Pre-empting the inevitable "30 seconds isn't that long you're not going to die waiting 30 seconds to see your cat videos" reply: Oh hold on, I'll just bin my SSD, and I believe Grace Hopper would like a word about nanoseconds. Chrome didn't need to waste those seconds, and so neither did Firefox. Unnecessary wastes of time are quite irritating.
The other issue was Chrome's excellent sync vs. Firefox's messy faff of one.
Two things have happened since:
1) An acute awareness of how much Google treat your personal info / metadata / browsing habits as an all-you-can-eat buffet, made all the more obvious in our recent privacy conscious times triggered by NSA et al
2) They finally fixed the Firefox browser.
There is still a foreground element to updates and their notifications, but nowhere near as obtrusive as they were when I abandoned it, add-ons don't usually break, and now it has a proper sync system.
Nowadays it's a no-brainer to use Firefox again. It gives you the same conveniences regarding sync and less annoying updates, but doesn't spy on you or restrict you like Chrome does and has much more respect for your privacy and freedom. Wouldn't touch Chrome with a bargepole now, and them locking it down even further only seals the deal.