Microsoft pushes an add-on to Firefox without asking permission.
Susabelle describes it.
Any Firefox user receiving a Microsoft update to either XP or Vista (and possibly Windows 7 but I cannot confirm this) in the last two weeks will have gotten this forced push, right into Firefox, and wouldn’t even have known it. If you go to Tools > Add ons, you should see an entry for Microsoft .net Framework Assistant 1.0. You will notice that you can disable it, but that the “uninstall” option is grayed out, meaning it is stuck there unless you do some fancy footwork to remove it.
This particular add-on is not something you want to have installed, in my opinion. Upon installation, it provides a ClickOnce capability that pretty much lets Microsoft do what it wants when it comes to your browser, as well as opening you up to all kinds of other nasties out there, since we all know the .net Framework is riddled with bugs that are not always fixed as quickly as they should be. The biggest security flaw with the ClickOnce install is that it allows easy installation of malicious software from websites, without your permission and knowledge.
Follow the link for more information and to find out how to remove this thing.
Aside: If regular readers wonder why I contemn Microsoft so much, this pretty much illustrates why.