At the moment the Jump to page function does not work with Firefox. Try it in your forum

The error is the if-check in the function "pageJump" in forum_fn.js:
if (page !== null && !isNaN(page) && page === Math.floor(page) && page > 0)

This is never true: page === Math.floor(page)
Because page is a String and Math.floor(page) an integer.

Solution 1: Do not check the type:
page == Math.floor(page)

Solution 2: Cast the String to Integer:
Number(page) === Math.floor(page)