come indicato da Support è necessario installare la nuova versione firejail_0.9.54~rc2_1_amd64.deb
e Firefox 60.0 funziona correttamente.
Support link:
https://firejail.wordpress.com/support/
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Sito GitHub bug tracker, link:
https://github.com/netblue30/firejail/issues/1939