If you take a look at the installed add-ons in Firefox you may notice an add-on called Microsoft .net Framework Assistant.
That add-on is different from most other installed add-ons: the first thing that you will notice is that you cannot uninstall it as the uninstall option is inactive in Firefox. You may also realize that you did not install that add-on actively which means it must have been installed by another application.
The Firefox Extension Guru posted a walk through on how to remove the Microsoft .Net Framework Assistant.
It basically comes down to the following steps (it might be a good idea to backup all data before proceeding).
The process is apparently a bit more complicated for Firefox 3.1 beta versions as the Microsoft .Net Framework Assistant is not yet compatible with that version of Firefox and thus not completely installed.
Update: The extension is automatically installed when Windows is updated either manually or directly using Windows Update.
The main purpose of the add-on is to support .NET application deployment. The .Net Framework Assistant for Firefox which is included with the .Net Framework 3.5 Service Pack 1.
It enables stand-alone "ClickOnce" applications to run from the Firefox browser.
Microsoft has published a support page that details how the .Net Framework Assistant can be removed from the Firefox web browser.
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