Sorry for the reply, but I already tried about:config xpinstall.enabled true

Hi,

You can also try right-clicking, Save Link As and install the downloaded file via Firefox File (Alt + F) > Open File or via Tools (Alt + T) > Add-ons, click the gear on the top right and Install Add-on from File.

The link is inside a webpage for an Avocent device, it doesn't list Save Link As as an option. Thank you though.

You can also try to inform them of the problem. It could be an error at the site and they may have not noticed and informing them would be helpful.

What type of Firefox add-on would a hardware device need?

Can you provide the page URL so we could look at the link?

It's a KVM device, so it has a viewer plugin