How does Firefox set the PRNG initial seed?

I am playing with the XORShift128+ function in Firefox. You can exploit it using 3 "randomly" generated numbers as seen in this cool post. But I can't figure out how it sets the initial seed aState0 aState1. Is it based on process start time?

My goal is to be able to specify a process start time and then use the XORSHIFT128+ function to iterate through a set number generation instances. This would be like using the first RNG after starting Firefox to find the next few like is done in the post above for the current seed. This is for a coding challenge but I'm not sure it's possible.