"prefs.js". It should be found under your profile in FF. Rename it to some other extension first. Launch FF and it should create a new file by this name with default values. Good luck.
How do I reset firefox back to the original settings when I initially downloaded it. I've messed around with the settings so often that I'm now need to start again as I failed to keep any notes on the changes I've made. (i know how incredibly stupid/lazy i've been)