Firefox Animation Control
The animations of stop-work repeat indefinitely, but they cannot be stopped, reset and started again unless you have an old version of Firefox. If you have version 56 or earlier you can use:
Apple Mac OS:
You can get version 56 from, for example, softexia.com.
Before you move this version into the applications folder, make sure you rename it to avoid deleting your current version.
I have tested version 56 with El Capitan (OS 10.11.6) and it works.
Windows and other OS:
You can get Firefox version 56 for other OS from the same site. However, I do not know if it is possible to install multiple versions, as it is with Mac OS.
If you can install this (or earlier? versions) then install the "toggle animated gifs" extension and use it to access this web site.
On my computer, refreshing the page resets the animations back to their starting points. That is better than nothing, but I cannot guarantee it will work for you.