why would anyone obsess about tracking down which account liked which post?
Normal people wouldn't. Unfortunately, there are a lot of assholes, stalkers, and people who are salty they got downvoted and want revenge.
Ever seen people on Reddit say "Whoever downvoted this, go fuck yourselves?" I can guarantee that, if they knew who downvoted them, they wouldn't keep their reaction contained to an edited comment.
Try going into the addons manager and searching for it from there. For me, it was one of the recommended addons, but I've got FF Nightly so I don't know if it's available on regular FF.