You can lock the related network.http prefs to prevent users from changing the settings.
That will only make it impossible to change the prefs on the about:config page directly.

• 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");

See:

• http://kb.mozillazine.org/Locking_preferences

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

Hi Thanks for the reply, however I have still not managed to lock the settings: Tools-Options-Advanced-Network-Connection Settings - and lock the settings to “use system proxy settings”. I have created a file: local-settings.js – local-settings.js is located in the same folder as the channel-prefs.js – There is one line of code in the file: local-settings.js - pref("general.config.filename", "mozilla.cfg"); However if I include "general.config.filename", in that file I receive popup an error on loading Firefox – Configuration Error – “Failed to read the configuration file”. If I remove - "general.config.filename", the program loads. Please advise. Also should the content of the mozilla.cfg file contain this code // lockPref("network.proxy.type", 0); Or should it be // lockPref("network.proxy.type", 5); or should it be - lockPref(); // lock pref, disallow changes. Many thanks Dave

If you get the Failed to read error then that means that there is a problem with the mozilla.cfg file or Firefox doesn't find the file as you may have created mozilla.cfg.txt in case you saved the file as a text file (check the type in the Properties via the right-click context menu to see if it is a CFG file).
The mozilla.cfg file should be in the main Firefox program folder where also the firefox.exe file is located.

You should have at least two lines in mozilla.cfg.
The first line should always be a comment (//) because this line is skipped.
The value of the pref depends on what you want to use for the connection settings, for use system proxy settings use 5.

//
lockPref("network.proxy.type", 5);

many thanks, cor-el that worked.

I ended up with this content in the local-settings.js file:

pref("general.config.obscure_value", 0); // only needed if you do not want to obscure the content with ROT-13

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