I figured out the reason: /usr/bin/apport-bug sets PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin but firefox is a snap living in /snap/bin/firefox.
I figured out the reason: /usr/bin/apport-bug sets PATH=/usr/ local/sbin: /usr/local/ bin:/usr/ sbin:/usr/ bin:/sbin: /bin but firefox is a snap living in /snap/bin/firefox.