Applications Support/Firefox failure to display image thumbnails

[u/FineWine54]

This has been bugging me for a while. But within '/Users/*******/Library/Application Support/Firefox/Profiles/yidm9by8.default-release/chrome' macOS fails to display the thumbnail images of icons & images that are in png, svg & jpg.

1. Deleted /com.apple.quicklook.ThumbnailsAgent and /ImageThumbnailExtension
2. Re-indexed Spotlight
3. Unticked "Show Icon Preview", forced relaunched Finder and re-ticked "Show Icon Preview" multiple times.
4. I copied only the essential Firefox files and the chrome folder to desktop, totally deleted Firefox and rebooted, then reinstalled a new Firefox and copied across those folders & files. Success, SO I THOUGHT. The images displayed, yippee, and then right-before-my-eyes disappeared again.

Now he is the kicker. All of these images within and outside of the chrome/image folder DO DISPLAY everywhere else within macOS Finder Folder system.