Rated 5 out of 5 stars

Sicilian Volovski কর্তৃক ফেব্রু. 2, 2017 তারিখে ·

It's a great plugin, but seems to not be maintained, so I had to dig for the patch myself.

You can get the patch here - https://github.com/SicVolo/firefox-custom-buttons/releases/tag/0.0.5.8.9.1

Please, note that it is not signed. I am not a firefox dev, just a code hacker who need to use the extension NOW. To make it work you'll need xpinstall.signatures.required set to false in about:config.
I've listed the changes to the code. If you don't trust the binary, get the code from https://github.com/SicVolo/firefox-firefox-custom-buttons
and compare to your current add-on yourself.

Hopefully the official patch comes soon.