This story came across my radar this morning (Microsoft Update Quietly Installs Firefox Extension). This seems bad news to me, and exemplifies one aspect of Microsoft's software model that I really rather dislike in comparison to, for example, how Ubuntu rolls out its updates.
Turns out this Firefox add-on, which is installed rather stealthily as part of a regular Windows .NET update, has its uninstall option disabled, which means Windows users need to go through a complex process to remove this, as described at annoyances.org (Remove the Microsoft .NET Framework Assistant (ClickOnce) Firefox Extension). Annoyances.org says: