Still working in FF 62.0.3 on macOS High Sierra 10.13.6. I would give 5 stars if videos did not play for a second (or a bit less) at the beginning. Allen, are you still working on this plugin?

It works! Sometimes it's a little bit annoying that the video plays for one second or so before pausing, though.

Sometimes it may take a second or two to auto pause but it never fails.
I don't know why other people are reporting that it doesn't work, maybe some incompatibility issues with other add-ons.