Removing uninstallable Firefox extensions
If you run the AVG install again you can remove the safe search. Most stuff can be found in your profiles folder if you want to remove it manually. This lives under your user profile application data folder under mozilla firefox profiles. The actual path depends on your operating system.
Full info here. What worked for me recently was to:
Delete (or rename) the following three files from the profile folder (the files will be regenerated upon application restart):
* extensions.cache
* extensions.ini
* extensions.rdf