Thank you for your bug report. That sounds like an issue with the firefox snap, opening a file in /tmp from a fileselector correctly works using the desktop portals but using a command line argument gives an error which is probably because it's trying to directly read from /tmp which is a private directory in the snap context
Thank you for your bug report. That sounds like an issue with the firefox snap, opening a file in /tmp from a fileselector correctly works using the desktop portals but using a command line argument gives an error which is probably because it's trying to directly read from /tmp which is a private directory in the snap context