I am a long time FF user, and have done a few extensions as well, and its not really the rapid release cycle that annoys me.
No, the constant change of the user interface, and decisions chosen by people who don't know anything about me or how I like to use my browser.
I cannot fathom why, when they change UI, they don't keep the "old" look in, and let existing users change to it, if they like, or stay in the old look, if they like that.
One of the largest bullet points in FireFox is that you can tailor the browser to your needs, via extensions, but somehow this doesn't extend to the most important part of the browser, or, any program for that sake, namely the UI.
THIS annoys me to no end.