I was looking in the librewolf FAQ and I have fond that:
In LibreWolf we decided to enable Enhanced Tracking Protection, as it plays nicely with uBO and it can block some extra scripts. (...) Finally, there's no point in changing from strict to any other mode, as strict mode doesn't usually cause any kind of breakage, and changing to custom mode to block cookies will come at the expense of disabling partitioning: not worth it, so we decided to hide the UI that allows users to change this setting. You can instead whitelist specific websites from the urlbar.
The FAQ redirects then to this comment on codeberg:
(...)users who change to other modes, do that for two main reasons, according mostly to reddit posts:
-they read strict so they think that if something is broken they need to change that. in my experience it never turned out to be true.
-they switch to custom to block third party cookies, which disables isolation making all the rest of the privacy measures useless, because lowest hanging fruit. even if they use FPI what they are getting is outdated code and more breakage, not good. (...)
Apparently disabling 3rd part cookies brakes the isolation, so they have decided to remove that option. I think that this is the best solution they were able to implement, but i have some perplexities. I wish they will solve this problem in the future. Of course, if anyone can clarify that point we'll appreciate it ;-)