Word of caution, if you have been browsing successfully until now, it could be a malicious javascript app or malware loaded from that website that is attempting to scan your network or do other things. In other words if this is a new firewall request above and beyond the standard one librewolf needs to function, proceed with cation.
My understanding is some parts have to be done sequentially even though the parts themselves are multithreaded, now the different parts can all be done in parallel.