You can look at these prefs on the the about:config page to see what settings work for you to block autoplay.

• media.autoplay.default = 5 [0:allow;1:blockAudible;2:Prompt;5:blockAll]
• media.autoplay.blocking_policy = 2
• media.autoplay.allow-extension-background-pages = false
• media.autoplay.block-event.enabled = true

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

• https://support.mozilla.org/en-US/kb/block-autoplay

Thanks for the info. Unfortunately, that did not work. The problem seems to be with specific sites that somehow are overriding these settings. Guess I just won't use them anymore.