Why is WU putting add-ons into Firefox? It's Mozilla's responsibility to take care of their own browser, and I imagine they would prefer it that way.
WU puts the new extension "Microsoft .NET Framework Assistant 0.0.0" into Firefox without any prior notice to user. I'm not sure offhand exactly which WU update does it, but apparently one of the High Priority security updates related to .NET 2.0/3.0/3.5. This add-on from MS is not compatible with Firefox. Only the Enable/Disable button is active, the Options and Uninstall buttons are grayed out.
MS tried to provide a fix to enable the Uninstall button, but it doesn't work with later versions of Firefox as I and many others can verify:
Update to .NET Framework 3.5 Service Pack 1 for the .NET Framework Assistant 1.0 x86 (KB963707)
MS also published manual removal instructions:
How to remove the .NET Framework Assistant for Firefox
I can verify that the manual removal method works OK with my Firefox 3.6 despite some inaccurate details. (There is no general.useragent.extra.microsoftdotnet preference item in my Firefox 3.6 about
:config list.) However removing it is pointless(!), because WU then reinstalls it via High Priority KB963707. The only way to prevent reinstallation is to disable KB963707 in WU.
In fact Mozilla has their own version of Microsoft .NET Framework Assistant 1.2 which works OK as I can verify with Firefox 3.6. It can be installed on top of the MS 0.0.0 version, then 0.0.0 doesn't appear. However the MS 0.0.0 version wasn't replaced and reappears if 1.2 is uninstalled. The MS 0.0.0 version is not needed, only causes problems, and somehow needs to be killed by WU.