Blocklisting is the ability to disable errant add-ons and plugins for all Firefox users. The current list of blocklisted items is published to mozilla.com

Blocklisting Policy

Block add-ons & plugins:
- with versions with known vulnerabilities or major user-facing issues
- with version ranges with known vulnerabilities
- with fatal bugs (client crashes on startup or something causing an endless loop of unusability)

Don't block:
- before we work with author/vendor to send out an update, so don't block the most recent version of a major addon/plugin
- for minor bugs or non-popular addons/plugins (crashes on event calls, messed up UI, etc.)
- if user has disabled compatibility checking and add-on causes problems/crashes
- plugins that are pre-release, alpha or beta

How to Blocklist an Add-on

file a Blocklisting bug
include extension guid, extension versions to be blocklisted and all affected appliction guids and version ranges
submit the bug and make sure to reference any related bugs

Testing the Blocklist Service

There are two ways to test the service:

update a local install of AMO with test entries and point extensions.blocklist.url at your copy of AMO
manually edit the local blocklist.xml in your profile directory to contain test entries
you'll want to adjust your app timer stuff using these instructions

More Information

XML definition
Current list of open Blocklisting bugs/requests
See here for additional details & information about blocklisting
Firefox 3 blocklisting - plugin-related open bugs bug 391731