You get this in case CSS files are blocked. If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, DuckDuckGo PE, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content. *make sure your extensions and filters are updated to the latest version *https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems 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.