Because Firefox only allows web extensions now, DownThemAll doesn't work with the latest version of Firefox. To fix this, you would just have to [http://ftp.mozilla.org/pub/firefox/releases/52.9.0esr/ install Firefox 52 ESR], which still supports legacy extensions. I recommend the Firefox ESR version because it can easily be installed alongside your regular Firefox release so that you can have multiple versions of Firefox running. Just make sure you pick a different installation folder so that you don't install over top of your regular Firefox. The project appears to have been abandoned by the developer because they can't get the same functionality from the web extension APIs. Mozilla recently removed legacy extensions from the add-on site, so can't download it from there anymore. You would have to [https://github.com/downthemall/downthemall/releases download the latest XPI file directly from their Github repository]. Hope this helps.