The cache folder in firefox contains files with a unique id as name and without extension, but those are actual images, videos etc. You can verify this by renaming some of them manually by adding '.jpg'.
To solve this you can make a simple script that iterates through all files in the directory, checks if the file is a 'webp' image (currently used by FH) and in that case copies and renames that file to an output directory.
I use something like that to get the images, it works well most times even though you'll also get false positives. This why I said above that getting the images from FH is much more painful than it used to be...
The 'about:cache' windows used to work well, you just needed to filter for entries pointing to the firebase of FH. After clicking on the entry link you would be able to see the image directly in the browser.
Now it seems this only works with older pre-2022 images (the ones that were sent without watermark), it seems newer images do not show up anymore :(
I am sure there are even more ways to get the images, perhaps much simplier than the one I described. People do not share them because they fear FH will patch it.
But once again, the problem is not getting the files, its the bloody watermarks on them, thats the real issue. Even if you get the images you cannot share anything with the watermarks on it or youll get banned by FH.