With Amazon on display, click on the site identity button (for details on what that is see https://support.mozilla.com/kb/Site+Identity+Button) and then on More Information. This will open up the page info dialog.

First select the Permissions panel, make sure that "Load Images" is set to allow (selecting Use Default should also work)

Next select the Media panel, then click on the first item in the list. Use the down arrow key to scroll through the list. If any item has the option "Block images from (domain name)" selected, de-select the option.

This should hopefully resolve your issue, but also see https://support.mozilla.com/kb/Images+or+animations+do+not+show.

Some add-ons can also block images, for example if you have AdBlock Plus installed, make sure that you have not accidentally created a filter to block the images.

Unbelievable. After a week of beating my head on this, I managed to find the problem, and why it is specific (in particular) to Amazon.com.

Amazon calls some of its images from a third party site, AND THE SITE IS UNTRUSTED! You can check/uncheck any pretty button, tab or menu item in Firefox, and the problem will not resolve.

I fixed this by:

Using the site identity button (for details on what that is see https://support.mozilla.com/kb/Site+Identity+Button) and then on More Information. This will open up the page info dialog.

Next select the Media panel, then click on the first item in the list. Use the down arrow key to scroll through the list. If any item has a non-Amazon URL, copy it into a new tab, putting it in the address bar, then go to the location.

Evidently, some of these are from an untrusted source. You may get the message box that says GET ME OUT OF HERE, or I UNDERSTAND THE RISKS. I made an exception for the site, and now all my images work again.

Now somebody needs to go back and fix the bug in the logic.