You can check the Web Console (Tools -> Web Developer) for messages about media content and about the presence of blocked/mixed/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.