According to about:memory µMatrix sandboxes all seem to live within the same zone (in process/remote tab child global, depending on e10s).

As I understand it this has a negative impact on GC performance since allocations by scripts loaded into those sandboxes triggers GCs in this fairly large zone instead of GCing in a window-specific zone. I would guess that the the cross-zone references are also be more expensive.

This can be mitigated by using the sameZoneAs option.

The addon SDK does this for its sandboxes, so it's probably best practice.