Flash block {web link} Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on it

Firefox has a setting to disable autoplay of HTML5 videos, but it doesn't switch off and on depending on whether a tab is foreground or background.

I wonder how Chrome does that... Is it only on YouTube, so possibly a script in the page that someone could dig out and analyze, or does it work on Vimeo and other sites as well?

There does seem to be a way for add-ons to detect whether a page is in a background tab or foreground tab, so it should be possible to create this missing feature if no one finds another workaround: https://developer.mozilla.org/docs/Web/API/Page_Visibility_API

I couldn't resist giving it a try. Since I don't write add-ons, I wrote a Greasemonkey userscript: https://greasyfork.org/en/scripts/22426-youtube-wait-for-me-pause-and-start-player-in-background-tabs

Unfortunately, the script isn't fast enough to stop the video from playing ever so briefly, I can't quite figure out Google's timing.

I just saw reference to a preference you might want to test out instead of the userscript or another add-on. I'm not sure why it is disabled by default; it might be a little buggy.

(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.

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

(3) Double-click the media.block-play-until-visible preference to switch the value from false to true

If you use this, let us know if it works well and/or has any strange side effects.