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]]?

You can check the Web Console (Tools -> Web Developer) for messages about blocked content and about the presence of mixed or unsafe content. *https://firefox-source-docs.mozilla.org/devtools-user/web_console/ You can check the Network Monitor to see if content is blocked or otherwise fails to load. *https://firefox-source-docs.mozilla.org/devtools-user/network_monitor/ If necessary use "Ctrl+F5" or "Ctrl+Shift+R" (Mac: Command+Shift+R) to reload the page and bypass the cache to generate a fresh log.