document.querySelectorAll('img').forEach((im) => { im.style.display = 'none'; })

Just press ESC and the gifs will stop

EDIT: Seems like I'm years behind https://bugzilla.mozilla.org/show_bug.cgi?id=825486

It's stop() now.

I write bookmarklets for things like this. Someone already posted some code below. They have the benefit of being quick to write, bespoke to my needs and low overhead.