Hi mickyneilan, is this the latest version of TrueKey? I ask that because Firefox recently changed how third party installers need to push their extensions into Firefox and probably only the latest version of TrueKey would be set up for that.

The error message you're getting can indicate a blocked redirect or problem verifying a certificate. What is the link you're using to install from? If it contains personal information (I can't really tell from the screenshot), perhaps you don't want to share that on a public forum. You could replace any unique codes with XYZ when posting.

Actually, no personal info is needed to get the extension from

https://www.truekey.com/en/onboard

The site pushes it automatically and directs Firefox to retrieve -- note this is a cross-site link since it comes from a different server:

https://downloads.truekey.com/firefox/latest/truekey.xpi

After you approve the installation, Firefox downloads the 6MB file.

When I look at the Browser Console, there's a warning, but it doesn't block me from installing:

1586930727334 addons.webextension.@true-key WARN Loading extension '@true-key': Reading manifest: Warning processing externally_connectable: An unexpected property was found in the WebExtension manifest.

If you try to access https://downloads.truekey.com/ do you get an error on that server?