I used Firefox (more specifically Iceweasel) as my sole browser from roughly version 3 until somewhere in the late twenties.

Recently I started using Chromium off and on because FF just seems to be so damn slow if I have more than four tabs open. Reddit + RES is so slow that it's completely unusable. I'm talking a several second delay between each keystroke and the character appearing in the input boxes.

Also, FF's sync is quite a pain to set up, especially if you don't have another device with you.

I fear the recent UI changes may have been the last nail in the coffin for me. I had a very nice setup using pentadactyl with some select add-on icons on the statusline. Now, that isn't possible. I have to choose between a superfluous navigation bar or no icons.