Is this with a secure https connection or with an open http connection.

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required)?

You can check the log in the Web Console for error messages and the Network Monitor (Firefox/Tools > Web Developer) to see if content fails to load or is possibly blocked.

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

Use Ctrl+F5 or Ctrl+Shift+R to reload the page and bypass the cache to generate a fresh log.

You can see a special padlock at the left end of the location/address bar.

• a padlock with a strike through means that mixed active content is blocked.
• a padlock with an exclamation mark attached means that mixed passive content (e.g. images) is present, but not blocked.

The shield icon, previously used for mixed active content, is now used for Tracking Protection.

• https://support.mozilla.org/kb/tracking-protection-firefox
• https://support.mozilla.org/kb/control-center-site-privacy-and-security-firefox

Sure. The page where I spotted the issue is: http://www.globalpropertyguide.com/ But the same effect occurs with any other web page with ads.

Not sure whether the http connection is secure or insecure. It's whatever you would get in normal home use. I have an anti-virus, Avast.

There's no padlock next to this particular url. There's a shield icon, as I'm now using tracking protection.

As to checking error messages via the urls below, I've taken a look but (for my expertise level) it looks complex, so I'm not sure that I'm interpreting things correctly. But so far as I can see, there are no error messages.

   https://developer.mozilla.org/Tools/Web_Console
   https://developer.mozilla.org/Tools/Network_Monitor

Does the Web Console ("3-bar" menu button or Tools > Web Developer) show any messages about content being blocked?

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

Not that I can see. But then maybe I am looking in the wrong place. When I open these pages that you pointed me to, there are a large number of sub-pages that I could look at, each with a lot of information. It is hard for the lay person to know what to look at.

For example the Web Console has the following sub-categories: Inspector Console Debugger Style Editor Performance Network Each of these has a second line with several sub-categories, particularly the second category, "Console". Network Monitor has the same categories.

So where to look?

Anyway, you have introduced me the Web Console, so I'll dig around and see what comes up.

Messages about content being blocked appear in Console. You can also see lines about files that are requested in both the Console and in the Network Monitor.

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

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