Thanks for your help.
I'm very sorry, but in doing what you asked, I found out that in fact the behavior is exactly the same with ubuntu and debian! There are always 3 processes. I never noticed that before, and I don't know why I had in mind I saw a different result with debian.
I suppose that this is part of the isolation firejail is supposed to provide?
Just for interest :-) here is the output you requested. They are exactly the same for ubuntu and debian
$ firejail firefox
Reading profile /etc/firejail/firefox.profile
Reading profile /etc/firejail/disable-mgmt.inc
Reading profile /etc/firejail/disable-secret.inc
Reading profile /etc/firejail/disable-common.inc
Reading profile /etc/firejail/disable-devel.inc
Reading profile /etc/firejail/whitelist-common.inc
Parent pid 12170, child pid 12171
Blacklist violations are logged to syslog
Child process initialized
1508426536368 email@example.com WARN Please specify whether you want browser_style or not in your browser_action options.
Thanks again and sorry for the disturb...