As of today, this and all addons are currently broken for FF. DO NOT UNINSTALL! There was a thread on Reddit that some sort of update or new system Mozilla was working on was implemented but while it certainly works to block installs... it was DOA so now all addons are broken till they either fix that new bit of software or w/e it is, or remove it. There is a work-around using the config, but I would not recommend it as it would not only fix addons, but open a backdoor through your browser, directly into your system. My 5 Star rating is based on how it worked before, not currently.

edit: one of the safer and VERY temporary fixes is to go into about:debugging and using it to "Load temporary add-on" if you uninstalled, right click the add to firefox button and click save to- put it anywhere then load about:debugging > Load Temporary Add-on > click the file wherever you saved it to. in most cases, this cancels once you close your browser so you'll need to reload it again.