Yep, and if open source licensing could be revoked on a whim, you can imagine the chaos that ensued. That would be my understanding as well, old version that have MPL license is perfectly fine to fork off, newer version might not be as it is under a different license. One of the reason why I liked Apache License is that it have make it explicitly clear that it’s irrevocable whereas MPL it is operating on an assumption that it’s not revocable. The most fundamental problem with the legal system in USA is that no law is “set in stone” and leaving things to assumption is open to reinterpretation by the judge who may have sided against you. (Hell, Google vs Oracle on Copyrighted API is still on case-to-case basis, so take it as you will.)
Disclaimer: I am not a lawyer. I just share what I learned from Legal Eagle youtube and few other sources.
That one was an old documentation that some of the Chinese folks actually document a lot of quirks related to X11 protocol. I paid about $6000 for translator to work on translating that doc to English and I use it to build my own GUI Toolkit on Linux that I still use to this day.