Try [[Clear cookies and site data in Firefox#w_clear-cookies-for-the-current-website|clearing site cookies]] and [[Enhanced Tracking Protection in Firefox for desktop#w_what-to-do-if-a-site-seems-broken|disabling Tracking Protection]] for the site. Does it still happen in [[Diagnose Firefox issues using Troubleshoot Mode|Troubleshoot Mode]]?

Thanks for your information. Found that I can load by pressing Cont+ F5 but it doesnt fix the problem and I dont know why it works

Does it still happen with default settings in a [[Profile Manager - Create, remove or switch Firefox profiles|new browser profile]]? An easy way to test a new profile is to install [https://aurora.mozilla.org Developer Edition] and see if it happens there or [[Refresh Firefox - reset add-ons and settings|refresh your existing profile]]. Make sure you stay signed out of your sync account when testing.