D’oh. The issue is with apparmor confinement. Disabling it for the browser "fixes" the issue:
sudo apparmor_parser -R /etc/apparmor.d/usr.bin.webbrowser-app ** Changed in: webbrowser-app (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1575556 Title: webbrowser-app fails to start when installed system-wide and run against a custom oxide build To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/webbrowser-app/+bug/1575556/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs