It's possible that Firefox's Enhanced Tracking Protection (ETP) is blocking something from running on the website, which will often trick the website into thinking that you don't have Javascript enabled. You can try disabling ETP on that website.

Otherwise, it could also be an extension that's blocking something from loading on the website. It's not uncommon for other content blockers (like ad blockers or privacy add-ons) to cause a similar issue. You can easily check to see if it's caused by an add-on by trying Firefox in Safe Mode.

Hope this helps.

Are you possibly using permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History) in case there is a problem with local storage ?