I woke up this morning and this is what greeted me:
A while back, Microsoft released the ".NET Framework Assistant" as a Firefox add-on. Today, Firefox blocked it to disable a security vulnerability that affects it. So if this happens to your Firefox browser, don't fret, it's just a security tweak.
Updated to add: The Mozilla Security blog has new information: "Microsoft has now confirmed that the Framework Assistant add-on is not a vector for this attack, and we have removed the entry from the blocklist."