(Windows 7: C:\Users\YOURUSERNAME\AppData\Roaming\,
  Windows XP: C:\Documents and Settings\YOURUSERNAME\Application Data (if I remember correctly) )

I had the (almost famous) problem, that, from one moment to the next, videos on YouTube would not play. I remembered, that I've already had the problem and reinstalling/disabling/enabling almost everything did not solve it, whereas creating a new profile did. The disadvantage of this, however is, that everything is gone and you have to completely reconfigure everything FF-related. By taking apart the profile, I found out, that the problem seemed to come from a corrupted setting. So, here's a method that retains your bookmarks, extension settings, etc... * Close Firefox * Go to your Application Data Folder (Windows 7: C:\Users\YOURUSERNAME\AppData\Roaming\, Windows XP: C:\Documents and Settings\YOURUSERNAME\Application Data (if I remember correctly) ) * Go to Mozilla\Firefox\Profiles * Open your profile folder (should be SOMESCRAMBLEDCODE.default) * Rename the files prefs.js and extensions.sqlite * Open Firefox * You get a new tab for every installed extension. Open each one of them and click ->only<- 'Allow Installation' and 'Next' * When you reach the last one, click 'Restart Firefox' after 'Next' When you restart, your theme and FF-internal settings are gone, but all the bookmarks, extensions, extension settings, sync, history, etc. are still there. -- Not much, but saves your from a lot of trouble I am aware, this is not a question, but if there is some sort of FAQ (I did not find ;) ), this could be added.