Firefox wore me out by version 15 with their rapid release insanity. That was about the time Firefox and Adobe Flash were crashing each other.
The upgrades were taking more time to get Firefox and the various extensions to work right than they were worth. I finally got things to work with version 15. I dread thinking about upgrading and having to deal with the incompatibilities and crashes with version 20 or later.
My experience since about then has been that there are far less incompatibilities with add ons than before.
Are your addons compatible with the next version of Firefox? Find out straight from the Addons window! https://addons.mozilla.org/en-US/firefox/addon/is-it-compatible/