Well, if you've updated recently that is.
To check, open Firefox, go to Tools > Addons. Extensions tab.
Note the Microsoft .NET Framework Assistant 1.0.
You can disable it but you cannot uninstall it. Unless you get creative...
Open disclaimer: I don't know the mechanics of what this update/addon does. I do know that .NET framework has security holes that make me very nervous to have it stealth-patched into the browser I use because I don't trust Internet Explorer. I also know techie computer people who use computers more than me that are likewise nervous. Which is sufficient for me to have disabled it immediately.