Firefox provides the "media.autoplay.enabled" setting, which can be disabled. It's a very difficult setting to implement correctly, but it works most of the time. The larger trouble is that many websites, including YouTube, expect autoplay to work and act slightly wonky when it doesn't.

Edit: Have some bug links

Original reporter suggesting the setting should apply to HTML5 media. Includes a very long discussion of the problem and why it's difficult: https://bugzilla.mozilla.org/show_bug.cgi?id=659285

Bug about websites that assume autoplay works, marked invalid because those websites shouldn't(?) make that assumption: https://bugzilla.mozilla.org/show_bug.cgi?id=1173848