Just to write as an answer what came up in comments :
Firefox stores its preferences (a.k.a. about:config screen parameters) in a file named prefs.js stored in each user home directory, in
~ represents the current user directory and
<profile_id> is a random generated string of about 12 characters.
This file is stored as a 'Delta' which means only parameters that differ from default configuration will be there. i.e. if default config is :
And your configuration is :
prefs.js will look like :
To do what you want, you have to either copy a 'default' configuration file to the user's when he logs (via its
~/.profile file for example) or simply replace the file with a symbolic link to a default one. (which would be read-only for users and read-write for admin, still an example)
Hope that helps.