You have to jump through several hoops to get rid of the crap Microsoft .net plug-in
1) Open your Start Menu and choose Run.
2) Type in regedit and press enter
3) Lookup the key: HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla\Firefox\extensions and delete the key there (Windows Vista 64-bit HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Mozilla\Firefox\Extensions).
4) Go into Firefox and type in about:config in the address bar
5) remove: general.useragent.extra.microsoftdotnet
6) remove: microsoft.CLR.clickonce.autolaunch