The current Nightly doesn't autoplay until I focus the tab now. I'm not sure if this is a Youtube-the-website feature or Firefox-lazy-load feature.

It's Firefox https://news.ycombinator.com/item?id=14824509

Is that a Chrome feature or Chrome-only Youtube feature?

Firefox has the same feature, though it is currently enabled by default only in the Firefox Nightly channel. You can use the `media.block-autoplay-until-in-foreground` about:config pref to enable it in the Beta or Release channels.

Thank you... I almost re-switched to chrome over this.

It's Chrome's lazy-loading of tabs. YouTube could presumably implement this on all browsers by detecting if the user is focused or not, but I don't know that it's always desirable.

I'm pretty sure it's a Chrome-specific lazy autoplaying of html5 video. I really like to alt-click Youtube links (so the page is loaded and queued), put my headphones on and switch over when I'm ready.

Firefox does this too. It's on by default for Firefox Nightly and an opt-in preference in the other update channels.

So what you say is that firefox does not do this. (unless you go deep in about:config which is highly discouraged by mozilla).

Firefox Nightly does, and it's not done by default in non-nightly (but you can enable it with "media.block-autoplay-until-in-foreground"). I imagine it will become the default eventually.

"Firefox doesn't do this" is not a valid one-sentence summary of what I just said.