Firefox has a feature to lock some settings so they cannot be overridden. This involves creating a couple of different files in Firefox's program folder. If the settings you want to lock are visible in about:config, you might be able to do it this way.

• Locking preferences - MozillaZine Knowledge Base
• Customizing Firefox - Autoconfig Files | Mike's Musings

Note that on Linux, the default is to use the "Use the system proxy settings" setting for the connection.

You can look at these prefs to lock:

• http://kb.mozillazine.org/network.proxy.type
• http://kb.mozillazine.org/network.proxy.%28protocol%29
• http://kb.mozillazine.org/network.proxy.%28protocol%29_port

Use a mozilla.cfg file in the Firefox program folder to lock prefs or specify new (default) values.

Place a file local-settings.js in the defaults\pref folder where you also find the file channel-prefs.js to specify using mozilla.cfg.

pref("general.config.filename", "mozilla.cfg");

These functions can be used in the mozilla.cfg file:

defaultPref();  // set new default value
pref();         // set pref, but allow changes in current session
lockPref();     // lock pref, disallow changes