Website runs slow on Firefox

Hi!

My website https://www.sunsara.pl has been running really slow on Firefox the last few days.
The problem occurs on different pcs and mac. The site works fine on all other browsers.
The main page loads fast but it goes very slow from there.
Every time I’m opening a page from the menu, I’m waiting for this script to load:

https://jamesroke.co.uk/js/jquery.min.php

Unfortunately I don’t know what that means.
I assume there has to be made some changes in WordPress. I have access and I built the page myself but I’m not a programmer, so be kind and use simple language ??