Users have reported problems with Yahoo News videos, but for most sites:

For Firefox 63

The old media.autoplay.enabled preference is obsolete. Mozilla is planning to add a media autoplay permissions section to the Options/Preferences page once the settings have completed full testing and interface design. For now, you can use what's available in this "work in progress."

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste autop and pause while the list is filtered

To set your preferred behavior:

Block autoplay by default

• Double-click the media.autoplay.default preference and set it to 1
• Double-click the media.autoplay.allow-muted preference to switch the value from true to false

=> Does that work for you?

Ask on a site-by-site basis (default for "Nightly")

• Double-click the media.autoplay.default preference and set it to 2
• Double-click the media.autoplay.ask-permission preference to switch the value from false to true
• Double-click the media.autoplay.enabled.user-gestures-needed preference to switch the value from false to true

Allow autoplay by default (default for 63)

• Double-click the media.autoplay.default preference and set it to 0 (or right-click > Reset)

To block ALL video, you probably could change a bunch of media settings but I've never tested it. What about audio? Disabling MP4 playback could affect audio on some sites...

Thanks, JScher2000. Although I'd like to disable all video, this seems to shut down the worst of the news sites for now. At some point, I'd like to know how to strip out video function entirely simply to avoid this entire portion of the web.

See also: Disable HTML5 Autoplay https://addons.mozilla.org/firefox/addon/disable-autoplay/

As noted above, the configurations suggested don't work on Yahoo videos, the most annoying of all!

So far, the Disable HTML5 Autoplay extension looks to be doing the trick, though!

Update: 2/26/19 - this extension does NOT disable autoplay on Yahoo videos!