Hey, was great till a week ago, now it doesn't appear on right-click drop-down menu, maybe because of Firefox updates? Update! I installed it again (over the previous one, without uninstalling and reinstalling) and now it appears...!
The Mozilla team is rolling out a fix for Firefox which should resolve the certificate issue affecting all add-ons. See their Twitter feed for details: https://twitter.com/mozamo/status/1124627930301255680