In February 2009, Microsoft released Service Pack 1 for version 3.5 of the .NET Framework. This update also installed Microsoft .NET Framework Assistant add-on (enabling ClickOnce support).
[Microsoft may be Firefox's worst vulnerability]

The update received a moderate amount of media attention after users discovered that the add-on could not be uninstalled through the add-ons interface.
[How to remove the .NET Framework Assistant for Firefox]
[Remove the Microsoft .NET Framework Assistant (ClickOnce) Firefox Extension]

Several hours after the website Annoyances.org posted an article regarding this update, Microsoft employee Brad Abrams posted in his blog Microsoft’s explanation for why the add-on was installed, and also included detailed instructions on how to remove it.

On 16 October 2009, Mozilla blocked all versions of Microsoft .NET Framework Assistant from being used with Firefox and from the Mozilla Add-ons service.
[Mozilla Blocklist]