extension not signed in Firefox 43+

[demoklion]

Starting with Firefox 43, add-on extensions and multi-item add-ons that include extensions need to be signed by Mozilla before they can install in release and beta versions of Firefox.

More info on mozilla's site

[polyzen]

Seems like you might have ran into this:
https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/

[vaeth]

Thanks for pointing out the firefox bug.

That being said: The extension which can be downloaded from the mozilla site is also signed by mozilla.

[demoklion]

bug in firefox, not in this extension. sorry!