The time was, in macOS in the browsers I used, that Command-. (⌘-.) would stop a GIF or other video from playing. Please add this functionality to Firefox, or if it already exists, please tell me what shortcut, if any, does this.
I’m not aware of any keyboard shortcut that can “Pause all” media playing in a page. Would be useful. I suspect there are extensions for this if you need it now.
Upon reflection, I could be wrong about about it regarding videos per se, but it definitely did exist for animated GIFs.