If you are a windows user having .NET Framework 3.5 SP1 installed and your Automatic Windows Update is ON, there is every possibility of a new extension in your Firefox browser named .NET Framework Assistant and the interesting thing is that this extension can’t be removed by simple means since the uninstall button is greyed out.
There is quite an uproar about this thing from Microsoft in the tech industry. You can find details at this link on how to uninstall this nasty little piece from your Firefox browser.
