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

• https://developer.mozilla.org/en/Tools/Web_Console

You can check the Network Monitor to see if content is blocked or otherwise fails to load.

• https://developer.mozilla.org/en/Tools/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.

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block (wanted) content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block (wanted) 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

Firefox shows the shield icon at the left end of the location/address bar in case content is blocked by Tracking Protection.

• click the shield icon for more detail and possibly disable the protection
• check the Web Console for more detail about what content is blocked by Tracking Protection
• "3-bar" menu button or Tools -> Web Developer
• https://support.mozilla.org/en-US/kb/tracking-protection-firefox

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

This problem appears to have occurred as a result of the recent Windows 10 update. A message was received in the notification area that graphics applications had been blocked in Firefox. Is this being caused by Windows or Firefox? The same version of Firefox on another machine running Windows 7 does not have the problem. Or is the problem with the graphics driver (which is up to date)?

Problem seems to have been solved by restarting Firefox!