A few month ago, Microsoft released a .NET Framework update that silently added an extension to Mozilla Firefox (If the user had Firefox installed).
In addition to installing the as advertised updates to the .NET Framework, it also installed the extension in Firefox without warning, and opened the independent browser up to some of the vulnerabilities of Internet Explorer.
I always encourage installing critical updates to software because they are necessary for security purposes. While .NET updates are not “critical,” they should not push add-ons for non-Microsoft software; especially without proper warning.
People who choose to use Firefox are generally not big fans of Microsoft’s browser for a reason.
The bright side is that the Microsoft add-on is incomparable with the Firefox 3.5b4, so if you want to rid your browser of Microsoft, upgrading to that version or just disabling the add-on are your best bets. Trying to delete the add-on requires editing your registry which isn’t worth the hassle in this particular case.