That extension is no longer available on the Mozilla Add-ons site. There are some discussions about that on the forum associated with the Add-ons site: * [https://discourse.mozilla-community.org/t/microsoft-net-framework-assistant-1-3-1-add-on-removed-by-adminstrator/4528 Microsoft .NET Framework Assistant 1.3.1 Add-on Removed by Adminstrator - Add-ons / Add-on Support - Mozilla Discourse] * [https://discourse.mozilla-community.org/t/microsoft-net-framework-addon-weirdness/4430 Microsoft .net framework addon weirdness - Add-ons - Mozilla Discourse] According to Microsoft, "The .NET Framework Assistant for Firefox is included with the .NET Framework 3.5 SP1." I suppose you could try reinstalling that service pack and see whether it adds the extension back to Firefox. In the following Microsoft support thread, that wasn't successful (and also that version would be so old that it is unlikely to pass the validation check): * [https://social.msdn.microsoft.com/Forums/vstudio/en-US/4ebf81f0-fc49-48a2-a9e8-42f0ebd1219f/microsoft-net-framework-assistant-131-addon-for-mozilla-firefox-signed?forum=netfxsetup Microsoft .NET Framework Assistant 1.3.1 addon for Mozilla Firefox (signed)] There are unofficial sources for the download, which you can scan with your usual security software and which Firefox will flag as unverified if it is not one signed by the Mozilla Add-ons team. This site looks fairly reasonable, although I have not tried it myself: * https://helpdesk.infoxpert.com.au/KB/View/2571-firefox-microsoft-net-framework-assistant--missing-from-addons-store