The Microsoft Windows Presentation Foundation plugin allows Firefox to view certain multimedia content on websites. This plugin was installed on many computers by Microsoft as part of a Windows update.
Recently, it was disclosed that this plugin has a major security vulnerability that can be exploited by malicious websites. Mozilla has added the plugin to the Add-ons that cause stability or security issues are put on a blocklist, which disables it in Firefox.
Mozilla has put the Windows Presentation Foundation plugin as an overridable blocklist item. This means that it is strongly recommended that you disable this plugin. You can do so multiple ways:
When you start Firefox, you may be presented with a blocklist popup similar to the following:
Make sure the Disable item is checked and click Restart Firefox to disable the Windows Presentation Foundation and restart Firefox.
You can also disable this plugin from the Add-ons Manager:
You can also disable this plugin file manually:
The security vulnerability is fixed by Microsoft's MS09-054 patch. If you need to use web content that relies on this plugin, and you have this patch installed, you can override the blocklist and enable the plugin.