As someone who dislikes previews (since YouTube adds that video to your watch history if you let it go for too long) this has been a must-have for me since YouTube keeps re-enabling previews in my settings.

simple and straightforward

Firefox needs restarting for it to work.
I was looking for a way to permanently disable previews and this worked.
The "video previews" switch in youtube's settings needs to be disabled per-device and also turns itself back on after a while(!).
And this add-on seems to disable previews permanently. Thank you!

Seems to work fine, very helpful, thanx!