The page returned by the URL about:cache-entry?... contains a line similar to this:
file on disk: /home/fviktor/.mozilla/firefox/7jx6k3hx.default/Cache/CF7379D8d01
This is the full path of the file cache, if any. I think you can read this file with Python as usual, but I have not tested it yet. The path name may also be none in the case of small files cached only in memory. You can get the contents of the file by parsing the hex dump on the returned page in this case.
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