I had this exact problem with Cookie5 and was hoping the new version would fix it, but sadly, no dice.
Cookie seems to work fine with Safari, but no website data whatsoever appears for firefox. If I look at about:preferences#privacy, there are many cookies there but nothing shows up in Cookie.
I have searched the forums and done all the fixes suggested in various threads for Cookie5 but no change. Yes, I have enabled full disk access etc etc etc, deleted various folders and files, and completely uninstalled Cookie5.
Please help! I suspect Cookie is looking in the wrong place or can't decode Firefox's data or something.
Thanks for looking into it.