I have a similar requirement and use a self-written script in Tampermonkey, in which I falsify the wrong time (GMT) to the correct time just for websites I have previously defined. Specifically, I overwrite the newDate function on these.
This means that Librewolf always displays the wrong time (GMT) by default, except for websites I want to have the correct time (which just are a few) without the need to disable RFP.
Yes, of course. You will have to adapt it to your time zone, as the German time zone is currently hardcoded. If I had known back then that I would be sharing this at some point, I would have tried harder. ๐