compat: why is eventCounts undefined?

https://developer.mozilla.org/en-US/docs/Web/API/EventCounts

window.EventCounts

Present in FF115+/ESR115, in all window modes, not affected by RFP - so why is TB undefined? AFAICT there is no pref for this - sounds like a compat issue waiting to happen

not a FPing vector (since all users are the same), but tagging it as such to keep track

cc @tjr @pierov