I wish Firefox would use PR_SCHED_CORE to reduce the likelihood of such leakage...
I would worry about cross site leakage. From my understanding that would be unavoidable as soon as you have more tabs open than cores, which feels like an unworkable restriction.
Imagine opening a 9th tab and bring told you need to upgrade your 3700X to a 3900X.
But that means essentially reserving a core for the browser only. I don't think that would be shippable by default.