Pausing based on time away from target tab rather than number of other-tabs open seems ideal to me. The Great Suspender uses this approach.
The two exceptions are audio (including backgrounded video) and messaging/notifications.
Just not... around.
Routinely, after both clean shutdowns and system crashes, firefox just loses all my pinned tabs. It's the primary thing that keeps me from using firefox as a default all the time. There's weird 'restore last session' and folder searching you can do to try to maybe 'recover' some, sometimes, but... WTH? I'm pinning things - I want them around. If I didn't want them restored on browser launch, I wouldn't 'pin' them. Unless pinning is intended to be transitory, and I just missed the memo?