It has been possible for more than a year to block all autoplaying videos in firefox, with and without sound, via a single setting using the about:config URL but with recent versions of firefox, other settings have been introduced. With the release of firefox 66, the new settings to change are:
media.autoplay.default = 1 (i.e. block for all sites by default)
media.autoplay.allow-muted = false (i.e. block video without sound as well)
They seem determined not to offer the ‘block everything’ option in the UI.