From MozillaZine Knowledge Base
While Firefox’s extension system is arguably the browser’s most powerful feature, it can also be a vector for malware. With Firefox 2.0, specific extensions can be blacklisted from a central server (by default, addons.mozilla.org). This preference determines whether or not Firefox will periodically retrieve a blocklist from the server specified in extensions.blocklist.url.
Possible values and their effects
Enable the extension blacklist. Retrieve the list from the server specified in extensions.blocklist.url at the interval specified in extensions.blocklist.interval. If blacklisted extensions are already installed, disable them, and prevent blacklisted extensions from being installed in the future. (Default)
Do not retrieve a blacklist and do not restrict extension installation.
First checked in
Has an effect in