Vlad
This is no a bug in the userscript engine.
This is a bug in Youtube.
Firefox did some afforts to handle this issue
the resolution of Performance.now() is about 5us as per MDN description
It depends on energy saving mode or etc
performance.now() is monotonic increasing. This is correct that two performance.now() can be the same
In Youtube, it use this timestamp to determine two history states are the same or not.
So people found that problem will occur in Firefox as two states are with the same timestamp
Orion should deal with it