This extension was working, however, after a certain amount of Firefox browser's updates it stopped working, and the developer didn't update the extension itself.
THIS EXTENSION IS FREE! The developer doesn't receive any money and doesn't benefit from maintaining it at all. IF YOU WANT THE EXTENSION TO WORK, DONATE SOME MONEY SO THE DEVELOPER WILL HAVE SOME MOTIVATION. The Chrome's version of extension works perfectly. FIrefox's version wasn't updated since 2016.