From MozillaZine Knowledge Base
While Mozilla’s add-on system is a powerful feature, it can also be a vector for malware. Starting with Firefox 2.0, specific extensions can be blocklisted from a central server (by default, addons.mozilla.org). In current versions, the blocklist is not only used for malicious extensions but also to disable vulnerable plugins & crash-prone graphic drivers. The information retrieved from the blocklist is stored in the file blocklist.xml. A current list of blocked add-ons can be viewed here.
Possible values and their effects
Enable the blocklist. Retrieve the list from the server specified in extensions.blocklist.url at the interval specified in extensions.blocklist.interval. If blocklisted extensions or plugins are already installed, disable them, and prevent blocklisted extensions from being installed in the future. (Default)
Do not retrieve a blocklist and do not restrict extension installation.
First checked in
Has an effect in