There must be something broken in your SP4 install. What exactly Firefox version have you installed - standard, EME-free, ESR?
Creating a new Firefox profile may be not enough - it won't fix your broken Windows user profile. You should better do what I suggest and create a new Windows user account. Otherwise the only way to remove all possible problems is to strictly follow steps described in FDM FAQ for Firefox troubleshooting.

Goof news - Bad news !

I did create a new user slate clean and FF bare with FDM and, FDM was unable to monitor in FF ! No plugins could be installed !!!

I then deleted that new user.

I resumed in my usual OS session in my usual fully loaded FF and tried another download to no avail ! same FDM behavior (un-) ! Even more : the FF own download manager that resumes after closing FDM wont work either !!!???

I then decided to closed the whole FDM program because of it would'nt want ot work properly and I suspected now that it could interfere with FF own download manager...

I tried another download and... SUPPRIZE ! FDM plugin kicked in right away and worked flawkessly !

So, I guess the solution amounts to : don't auto start FDM at window startup ! the FF plugin will still be active and ... "take care of business" !

Best regards,

iBenny