Any possibility this is a side effect of Bluhell Firewall? Those gallery overlays might look like advertising.

To determine whether one of your extensions is involved, you could test some of the same pages in Firefox's Safe Mode. That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled (Flash and other plugins still run)

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

Thank you for your help. When started in safe mode the problem is gone. What do I do now? Do you think it is the Bluhell Firewall? How do I turn it off. It isn't listed in the add-ons.

Thanks again.

There is (supposed to be) a toolbar button for Bluhell Firewall, so you could try turning it off and reloading the page to see whether that allows you to see the images. If not, then something else is causing the problem.

If you do not see the toolbar button, it may be among the unused buttons in the Customize dialog. This article shows how to move that out onto a toolbar: Customize Firefox controls, buttons and toolbars.

Forgot to mention. The Add-ons page has multiple categories. In this case, to selective enable/disable different extensions, you will need to click Extensions in the left sidebar.

You might want to try disabling InvisibleHand, as recent reviews suggest it can slow page loading.