background/defocused tabs are ‘paused’ by default.
paused meaning no runtime execution of scripts or anything else.
firstly, there’s always some security and plenty of privacy mischief around focus.
secondly, it’s almost always wasting cycles, so its just wasteful of resources and energy.
ofc with some option for you to eg. right-click on a tab and mark it as ‘runtime in background’ or something, for webmail or messengers etc which you do want runtime.
but it should essentially be whitelisted.
i’ve actually played with this in the firefox debugger and it essentially appears feasible so really hope this feature comes oneday - or i finally get some time to look into making an addon for it.