HI, could your security software be blocking anything ?

Have you tried Firefox in Safe Mode ?

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

In Firefox Safe mode these changes are effective:

• all extensions are disabled (about:addons)
• default theme is used (no persona)
• userChrome.css and userContent.css are ignored (chrome folder)
• default toolbar layout is used (file: localstore-safe.rdf)
• Javascript JIT compilers are disabled (prefs: javascript.options.*jit)
• hardware acceleration is disabled (Options > Advanced > General)
• plugins are not affected
• preferences are not affected

TEST''''is issue still there ?

As the next step will possibly be a Refresh.

With a Legacy extension this message would suggest that the extension's XPI file needs to be unpacked. I don't know whether this still applies to WebExtensions. Dictionaries are another example of a file that needs to be unpacked.