I have searched a bit more. The transition is in process since 2023. [1] I don’t find any announcement though.
It looks like the process is now finished and the github repository is now the official main source. There are other news sites claiming this. [2]
[1] https://glandium.org/blog/?p=4346 [2] https://www.phoronix.com/news/Firefox-On-GitHub
Happened to my wife yesterday. Some update broke grub.