Assuming that FF attacks can be designed like this:

Public display of action (single school shooter, bazooka aimed at capital building, airplane into building)

+

Hidden destructive devices to ensure maximum effect (back up shooters, submarine launched missile, explosive charges placed in towers)

=

Shock and Awe.

*Then isn't it possible that Notre Dame was supposed to go down like this:

Car bomb goes off

+

Internal fires light at same time

=

Car bomb burned down entire Notre Dame Cathedral = Muslum vs. Christian war breaks out in Paris. (ultimate FF goal)

So it was NOT an "accident" and it was NOT moooslims. It was a FF that was mostly stopped, at least the optics part to spark the war was disabled.

Remember, the only way to hide their crimes at this point is to start WW3.