After some investigation I believe that the issue is linked with my other one: [4.2.0.21] [X10 II] Video player overlay missing from videos
I suspect that xdg-open is not fully adapted to jolla-gallery being run via firejail. My believe is that xdg-open can’t access the already running jolla-gallery process and also can’t spawn a new one since it is being run as single process:
/usr/bin/invoker -s --type=generic /usr/bin/sailjail -p jolla-gallery.desktop /usr/bin/jolla-gallery
I filed a ticket in jollas internal bug tracker for this issue.