From time to time on my old-winxp-hdds i must remove the netframework-assistant Firefox-Plugin.
But there are unclear instructions:
1. deleting the content and not the folder DotNetAssistantExtension like in german instruction!! so fra so clear.
Start Firefox. The add-on should not be in the list anymore.
2. method two step 2
In the Firefox user agent, reset the changes that you made in step 1. To do this, follow these steps:

Start Firefox, type about:configin the Firefox address bar.
Scroll down or use Filter to find the following preference item:
Right-click this item, and then click Reset.
Restart Firefox

Anyhow since i dont have
it doesnt bother me.
3. I have the netframework assisstan 1.0 update. if that is what i need.
the whole purpose is about netframework assistant firefox addon to be greyed out and cannot be removed.
here the MS-explanation:

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.

Install the update to .Net Framework 3.5 SP1 which installs the add-on on a per-user basis which enables the uninstall button in the Firefox Add-on Manager.
Remove the add-on by deleting the {20a82645-c095-46ed-80e3-08825760534b} subkey in the Windows Registry. You find it for 32-bit systems in HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla\Firefox\Extens ions and for 64-bit systems in HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Mozilla\Fi refox\Extensions.