The media.autoplay.default setting (value=1) in about:config used to prevent video/media from playing automatically on loading a page with such content present.
Since the recent (2019-03-25) upgrade to 66.0.1+build1-0ubuntu0.18.04.1 video just plays automatically regardless of this setting.
I have a no flash/java environment, so we're just talking HTML5 here.
There are other media.autoplay options available, but setting them doesn't seem to fix the problem completely and breaks other aspects of the browser.
Does anyone know a fix to stop media autoplay in Firefox?
media.autoplay.default;1 media.autoplay.block-webaudio;true media.autoplay.allow-muted;false
And in 66+, also add: media.autoplay.enabled.user-gestures-needed;false