Make sure you are not blocking content.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

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

One issue that seems more common lately is Firefox's Tracking Protection feature. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature: Tracking Protection {web link}

Thanks, yes works fine in Safe Mode but my ad blocker etc disabled. Tried to turn on tracking only in private mode, should I remove all tracking options to display mixed content in normal mode?

Hi, yes you should as should not run for long in Safe Mode.

So if it was the adblocker then turn it off or adjust it.

Leave tracking Protection for only in Private is correct.

It could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Unfortunately Hardware Acceleration does not exist in Firefox V60.0.1 (64 bit) I could view emails ok on a slower Acer machine in WIN10 but not cannot since Firefox upgrade on faster Dell machine with WIN10.

Thanks Pkshadow, I disabled Ghostery which is the Mozilla Firefox suggestion in Firefox V60.0.1 and it fixed the problem, I will try and revert to the ad blocker recommended by Mozilla previously.