If you use extensions (Firefox/Tools > Add-ons > Extensions) that can block content (e.g. Adblock Plus, NoScript, Flash Block, Ghostery) then make sure that such extensions aren't blocking content.

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

Thanks, but this isn't the problem. Turning off Adblock doesn't change anything. I just tried. Plus I have used Adblock for a long time and never had a problem until a few weeks ago and, as I say, it's only happening on some websites.