Comment 6 for bug 1977782

To answer @me-again's question: if one uses the File menu->Open file facility in an attempt to open a file in /tmp, then Firefox copies the file to a newly-created directory in the /run/user hierarchy, and opens that new copy of the file with a file:// URI.

This problem appears to be a side-effect of some sort of attempt by the Firefox developers to mitigate the security vulnerability described at <https://thehackernews.com/2019/07/firefox-same-origin-policy-hacking.html>.