I tried your link using several versions of Firefox with no issues.

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem,

Diagnose Firefox issues using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

While you are in Troubleshoot(Safe) mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

Thanks, FredMcD, for the reply. I have tried all the steps you suggest but the issue remains unresolved. I find it strange that, while both Firefox and Edge showed exactly the same issue, while disabling hardware acceleration in Edge resolve the problem for Edge doing so in Firefox has no effect. Interestingly I tried the site in Internet Explorer and got a flashing screen that was again resolved by disabling the hardware acceleration (use software rendering instead of GPU rendering). So disabling hardware rendering fixes the problem in Microsoft browsers but not in Firefox. Intriguing.

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

If the problem goes away with the computer in Safe Mode, the problem is with one of the other programs on the system.

As a test, disable your protection programs.

Thanks for the reply. Triggered by your previous reply, I tried blocking (rather than clearing the cookies and cache) the cookies on the example problem site. This stops the screen flashing in Firefox. So for Firefox the screen stops flashing with a block on cookies from the site but with hardware acceleration enabled. For Edge/Internet explorer the screen stops flashing with the opposite settings cookies enabled and hardware acceleration disabled. A bit of a hack solution but it will do for me.

I called for more help.