I found the solution.
The problem was due to the wrong installation of firefox on my debian server.
After adding the good repository deb http://ftp.fr.debian.org/debian sid main
I was able to install firefox-esr and its dependencies via a simple apt install command.
Once all was installed, the python script worked as expected !
The new problem for me is now to configure www-data user permissions in order to allow it to execute the python script which needs write and execute permissions.
(Currently encountering
IOError: [Errno 13] Permission denied: 'geckodriver.log'
)
I am looking for a descent solution: allowing its execution as root without password seems too dangerous in terms of security for me..