Excellent extension! Using it on on Fedora 29 on FF 66.0a1 Nightly as well as other platforms and versions in the past. Shows resolution and tooltip shows URL - important for the site I download from.
Regarding other reviews saying it doesn't work on youtube, no it does not. But I don't expect it to, youtube has very specific video streaming methods and requires constant updates to keep up ith the changes google makes to the youtube system. For youtube I use https://addons.mozilla.org/en-US/firefox/addon/youtube_downloader_webx/
But for thousands of other sites and videoplayer software, this extension works amazingly! And the extensions that do work on youtube, don't work on most other sites, in my eperience.
I ill be trying out some of the other extensions from this developer!