Good news… Eventually, I was able to run firetools, but it doesn’t fix my problem.
I tun a torrent client in firejail (ktorrent), and when my torrent is downloaded, I try to export the corresponding file in my sandbox the the real filesystem with:
firejail --get
but firejail refuses the operation when my file name includes no alphanumeric character such as ‘[’
Switching to pid 15908, the first child process inside the sandbox
Error: invalid X11 display range
Changing root to /proc/15908/root
Child process initialized in 2.13 ms
Cannot start application: Permission denied
I do not know much about the tool you try to use, but please note that it is best to post what you see in the terminal by copy/paste of the complete action (that is: line with prompt and command, all output and line with new prompt) as Preformatted text (the </> button in the tool bar of the post editor (see also Posting code or preformatted text - Using Discourse - Discourse Meta ).
When you do that it will probably unveil who the “I” is that you describe above. The system does not know any “I”. It knows users normal users and root and we have no idea who is doing things.