[TomGuy]

I had the same issue in Firefox.

One solution is to edit the Firefox config:

On a blank Firefox webpage, type into the address bar: about:config.

Search for xpinstall.signatures.required true

Change to xpinstall.signatures.required false

[KevinB]

Change to xpinstall.signatures.required false

I tried that and it did not work for me. Installing the updated extension files by Pollard and linMcHlp linked in post 28 is the only thing that worked.