Raspberry Pi boot automation: systemd service + Firefox kiosk mode -- 2

Project Description

YOU NEED TO HAVE A Rasb PI for testing!

I am working with a Raspberry Pi that runs a custom Python application called TheBlackBox.
I need an experienced Linux / Raspberry Pi developer to fix and harden the boot automation.

What should happen on boot

The Raspberry Pi boots into a graphical environment (X11).

Firefox ESR starts automatically with Marionette enabled.

Firefox opens fullscreen / kiosk mode and displays:
http://localhost/theblackbox/theblackbox.html

After Firefox is ready (Marionette port 127.0.0.1:2828 is listening), the Python backend starts automatically:

sudo python3 /opt/theblackbox/theblackbox.py -d


No user interaction is allowed or required (true kiosk setup).

Current situation / problems

I already use systemd services (firefox-marionette.service, theblackbox-browser.service, theblackbox.service).

Services sometimes start in the wrong order.

Marionette (port 2828) is not always available in time.

Firefox sometimes does not open visibly on the screen after reboot.

The Python script works when started manually, but not reliably on boot.

Ordering cycles and timing issues occur with graphical.target.

What I need from you
Diagnose and fix the systemd services
Ensure correct dependency ordering (X11 → Firefox → Python backend)
Make the setup 100% reliable after reboot
Firefox must always open fullscreen in kiosk mode
Clean, maintainable solution (no hacks)
Environment
Raspberry Pi OS (Debian-based)
Firefox ESR
Python 3
Apache (localhost web interface)
X11 (not Wayland)

I have the total files

Deliverables
Corrected systemd unit files
Boot-tested and stable solution

Required skills
Raspberry Pi / Linux
systemd (advanced)
X11 & DISPLAY handling
Firefox kiosk automation
Python service integration

Please write Banana in your response so i know that you have read everything.