You could double-check possible content blockers. Firefox's Tracking Protection feature, and extensions that counter ads and tracking, may break websites that embed third party content (meaning, from a secondary server).

(A) Do you see a shield icon toward the left end of the address bar, near the lock icon? Clicking that should provide the option to turn off blocking for the page. More info on managing the Tracking Protection feature in this article: Enhanced Tracking Protection in Firefox for desktop (before Firefox 70: Content Blocking).

(B) Extensions such as Adblock Plus, Blur, Disconnect, DuckDuckGo Privacy Essentials, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix should provide toolbar buttons to manage blocked content in a page. There may or may not be a number on the icon indicating the number of blocked items; you may need to click the button to see what's going on and test whether you need to make an exception for this site.

When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that.

In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites.

Then try reloading the page. Does that help?

It has absolutely no effect, neither on a ubuntu pc nor on an iMac.-( Even if it did work it would a rather awkward way to operate. (Just for the record, storage clear, adblock plus off.)

Correction: Storage, cookies and adblock do not seem to be involved, but it does seem to be firefox tracker-blocking that is causing recent (since about July 2020) problems with the non-loading of sellers' images at #gumtree and #ebay . A cynic might hazard a guess that support at gumtree etc. know exactly what is going on despite dismissing it as a miscellaneous mystery #firefox issue. And perhaps gumtree and ebay are serving up different tracker-ware dynamically on-demand which might explains the strange intermittent nature of the problem? (And also maybe why 'view-image' of a missing (blank) thumbnail-image in a seller's ad would then load the thumbnail correctly?)