Follow these steps to disable nearly all video playbacks in Firefox.
To disable non-Flash Player video:
Type this into web address and press Enter: about:config
Click the “I accept the risk” button when this warning appears.
Right-click on the following lines under media prefixes to set the value to False:
media.autoplay.enabled
media.directshow.enabled
media.eme.enabled
media.encoder.webm.enabled
media.ffvpx.enabled
media.gmp-eme-adobe.enabled
media.gmp-widevinecdm.enabled
media.mediasource.enabled
media.mediasource.mp4.enabled
media.mediasource.webm.enabled
media.mp4.enabled
media.navigator.enabled
media.ogg.enabled
media.peerconnection.enabled
media.peerconnection.video.enabled
media.play-stand-alone
media.raw.enabled
media.wave.decoder.enabled
media.wave.enabled
media.webm.intel_decoder.enabled
media.webvtt.enabled
media.wmf.enabled
To disable Adobe Shockwave Flash video, click the “Open Menu” button at the top right part of the screen, then select “Add-ons”.
Select “Plug-ins”, then change the Shockwave Flash setting to “Ask to Activate” or “Never Activate.”
Non-Flash video playback can be disabled in the Chrome browser by installing a free add-on such as Disable HTML5 Autoplay.
To see all Firefox About menus, type this into the web address and press Enter: about:about