Microsoft .NET Framework Assistant 1.0
Microsoft took a step too far recently when they installed an add-on into Firefox in a recent Windows update "Microsoft .NET Framework 3.5 Service Pack 1". As you can see in the following image I have disabled but not been able to uninstall the add-on.
The reason it can't be removed is that Microsoft installed the add-on for all users but the Firefox GUI can only handle uninstalling add-ons for the current user. Microsoft have released a further update that installs the addon on a per user basis rather than for all users which enables you to uninstall it via the previously inactive uninstall button in the Firefox GUI. The download can be found here.
The reason it can't be removed is that Microsoft installed the add-on for all users but the Firefox GUI can only handle uninstalling add-ons for the current user. Microsoft have released a further update that installs the addon on a per user basis rather than for all users which enables you to uninstall it via the previously inactive uninstall button in the Firefox GUI. The download can be found here.