Hi, I am here to tell you that it is not particularly trivial to make the kind of changes required to make the websites keep working while also preventing stuff similar to JS fingerprinting.
Some extensions do a decent job in certain cases, but the only ones that completely fix the problem are the ones that simply turn off JS. I checked out what Librewolf's changes do, using amiunique.org and in some tests it even ends up increasing the uniqueness.

You will essentially require identifying different parts of the JS engine that expose said vulnerabilities and then creating mitigations for each of them, with either the "blend in" or "randomise" strategy and will also require to make sure they are not detected over any domain (due to partial overlap of either change).

This kind of change for a single person will require properly understanding the JS engine codebase and then making and maintaining all required patches over the course of the fork as the main project goes forward. This is pretty much a full time job.
Even if multiple people are working on it, one would still require a good understanding of the codebase.

I suggest recruiting one of the retired/laid-off Firefox engineers, if you have the funds.