The developer removed it because they did not like the new requirement of the Mozilla Add-on's site that developers add Multi-Factor Authentication to their Firefox Account: https://github.com/fanfare/googleimagesrestored/commit/8e77eb0e4314df9a64c8d3df2eb04c9868795085 Perhaps they would allow someone else who doesn't mind the requirement to repost it under their account?

By the way, this alternative popped up in a search, but I haven't tried it myself: https://addons.mozilla.org/firefox/addon/view-image/