they didn't create a fork under their control
I'm sorry but this is simply incorrect (See 1,2,3), as I have previously stated. You could point to sources that agree with you though if you disagree.
1: https://itsfoss.com/librewolf/
2: https://wiki.gentoo.org/wiki/LibreWolf
3: https://lwn.net/Articles/1012453/
These are some examples that use "fork" in describing Librewolf.
What they are doing is customising the current code of Firefox at the time of compiling the LibreWolf project.
You have described the creation of a fork.
... I'm not going to continue a fruitless argument.
I'm here if you wish to discuss further.
At which point it simply gets forked from the last release/commit. Unless the project nukes all of the history and no one comes forward with a backup.
And, they would typically need the consent of every contributor to legally change the license, unless they had been forcing contributors to sign over rights to their code before contribution.