If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (e.g. Adblock Plus, NoScript, Flash Block, Ghostery) then make sure such extensions do not block content. There is security software that can interfere with loading content, so check that as well.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• Do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Problem was solved by disabling the add-on : No-script suite lite I have used this add-on for months without a problem and after trying all my log on sites I found only two that were affected.