Thank you for responding, in the meantime I found a way to fix my little problem.

In fact, the Freelancer version I found on an abandonware site (since my original CD version didn’t want to work at all on win 10) had a built-in FLMM, version 2.0. I tried to reinstall an older version I still had in my archive file on my computer (version 1.3) and copy-pasted the mods in the corresponding folder of the old version… and the Just For Fun mod 3.0 worked like a charm, like all the other mods.

Anyway, thanks again for responding to this post, feel free to delete or close it since the problem is solved 🙂