Could you please check the error log on the frontend (F12 opens the console, the refresh), if it shows anything. Mine was empty, didn't show any errors unfortunately.
Also, the your system specs, operation system version, screen resolution, list of plugins installed in firefox might help.
Are you logged in by any chance? Is the same if you are not logged in on your site? WordPress outputs different html for logged in and non logged in users.
In the meantime I tried multiple web proxies, another computer, 5 VPNs but it's still ok for me. I honestly have no idea what's the difference and why is it happening on your side.Best,
If you like my products, don't forget to rate them on codecanyon :)