If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox shows a purple shield instead of a gray shield at the left end of the location/address bar in case Enhanced Tracking Protection is blocking content. You can click shield icon for more detail and possibly disable the protection. You can check the Web Console for messages about blocked content.

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Remove the Cookies" for websites that cause problems:

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Clear Data -> Cached Web Content: Clear

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 -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

cor-el,

I don't use any extensions. I have Enhanced Tracking Protection set on Standard.

I can get the eBay thumbnail images to appear on the https://outlook.live.com website, by clicking on the purple shield and turning off Enhanced Tracking Protection.

Something must of changed in version 79.0 because the images were not blocked before.

I was trying to add the website to the Manage Exceptions window, but I can't figure out how to add the website.

If you disable Enhanced Tracking Protection via the slider in the shield icon then this should automatically create an exception that shows in "Options/Preferences -> Privacy & Security". That is how you create an exception as the exception that shows in "Options/Preferences -> Privacy & Security". That is how you create an Exceptions dialog only allows to remove an exception.

In order to add the website, I had to use the regular browser. It will not add websites when using private window mode.

The thing is, I still have to turn off Enhanced Tracking Protection, with the browser in private window mode to get the thumbnail images to appear.

I guess I'll just toggle Enhanced Tracking Protection off and on, when viewing eBay email notification thumbnail images.