I just noticed that I have a Firefox add-on installed that I have no recollection of ever installing. This is the Microsoft .NET Framework Assistant and it appears to have been installed back in February 2009 as a part of .NET Framework version 3.5 Service Pack 1.
When I first saw this add-on, I was a little worried, and my first reaction was to attempt to disable or uninstall it. However the "Disable" and "Uninstall" buttons were greyed-out. Needless to say, this has been portrayed in a rather sinister light out on the web. For example, see here. Also, note that someone has added the tags "backdoor" and "malware" to the page at MSDN that describes this add-on. However, Microsoft insider, Brad Adams says here that the greying-out was down to an oversight by the .Net Framework team:
We added this support at the machine level in order to enable the feature for all users on the machine.Seems reasonable right? Well, turns out that enabling this functionality at the machine level, rather than at the user level means that the "Uninstall" button is grayed out in the Firefox Add-ons menu because standard users are not permitted to uninstall machine-level components.
The good news is that the latest update to the .Net Framework, which was installed on my machine this morning, re-enables the "Disable" and "Uninstall" buttons on the .NET Framework Assistant add-on, so you can now disable or remove it if you wish.
However, it still remains rather worrying that Microsoft should think it OK to silently install stuff into a rival company's browser, when that stuff could adversely affect the operation of that rival company's browser.