Today I've discovered the great Safari Theme for Firefox by Ran Aroussi and obviously I've decided to download the .jar and to install it.
The installation was ok, but when was the time to restart Firefox to apply the new setting.... crash!!! Firefox was unable to restart for a missing component and the browser screen was totally white and unusable. 
So... what to do? Uninstall Firefox and restart with a fresh installation? No no no, remember that this is Firefox! 
So, I've opened the folder where Firefox stores all the user settings (on my system C:\Documents and Settings\Stefano\Application Data\Mozilla\Firefox\Profiles\default.o3g) and I've opened the prefs.js file with Notepad.
On this file you can find all the user preferences. Just go to the line related to the skin configuration:
user_pref("general.skins.selectedSkin", "SkinName");
(where SkinName is the name of the current skin) and change the name of the wrong skin with your previous skin. When you restart Firefox all is ok... 