(note:before doing step one, firefox should be closed)

1. Locate C:\WINDOWS\Microsoft.NET\Framework\v3.5\Windows Presentation Foundation\DotNetAssistantExtension

2. Delete everything in the folder.

3. Type about:config in the firefox address bar. Under filter type: "general.useragent.extra.microsoftdotnet" (without quotes)

4. Right click on the entry and select reset.

5. Restart Firefox.