To this, have you tried running a no-autoplay style extension? (Not exactly this scenario, but lists might be modifiable)

I expect YT is probably doing some preloading or heavyweight running in a loaded-but-not-playing tab.

Convert that to load-on-click, and you'll likely fix your issue.

No need – Firefox does that out of the box for tabs opened in the background (I open tabs via middle/command click)! So there is no problem for me.

We’re shifting targets here.

The fact is, it doesn’t happen in Chrome for “magical” reasons, according to a comment few levels up. And regular non-tech users won’t debug this with click to play or whatever. We expect that YT does it absolutely intentionally.

It bends userbases however it wants due to its multi-dimensional reach, and we’re like “hmm maybe extension would help prevent accidental preloading issue”. I just don’t get it.