Displays additional gallery information when hovering over thumbnails. Only works in thumbnail mode.
Should be fixed now
Doesn't work with latest greasemonkey in Firefox 57.
Pretty much nothing works with the latest Greasemonkey since they changed/removed most of their APIs without proving backward compatible alternatives.
Consider switching to Tampermonkey or Violetmonkey.
Firefox
This script worked with Firefox somehow, until the new update, now it doesn't.