The proxy setting is stored in the user's prefs.js file in their Firefox profile.

The path to the Firefox profile directory and the file is:

%APPDATA%\Mozilla\Firefox\Profiles\7b9ja6xv.default\prefs.js

where "7b9ja6xv" is a random string. However, the directory of the default profile always ends in ".default". Most of the time there will be only one profile anyway.

Setting you are after are named "network.proxy.http" and "network.proxy.http_port".

Now it depends on what technology you are able/prepared to use to change the file.

P.S.: If this is about changing the proxy settings of a group of users via the logon script or similar, I recommend looking into the possibility of using the automatic proxy discovery (WPAD) mechanism. You would never have to change proxy configuration on a user machine again.

I don't think you can. What you can do, however, is create different profiles for each proxy setting, and use the following command to switch between profiles when running Firefox:

firefox -no-remote -P <profilename>

The easiest way to do this is to configure your Firefox to use a PAC with a file URL, and then change the file URL from the line command before you start Firefox.

This is the easiest way. You don't have to write a script that remembers what path to prefs.js is (which might change over time).

You configure your profile once, and then you edit the external file whenever you want.

This is the final compiled solution which worked for me... Tried and tested...

Steps to Change Proxy settings in Mozilla Firefox via cmd in Windows

1. cd /D "%APPDATA%\Mozilla\Firefox\Profiles"
2. cd *.default

To Replace Already Present Proxy Settings with User-defined ones

3. powershell -Command "(gc prefs.js) -replace 'user_pref(\"network.proxy.http\", \"already present IP\")\;', 'user_pref(\"network.proxy.http\", \"your http proxy ip\");' | Set-Content prefs.js"
4. powershell -Command "(gc prefs.js) -replace 'user_pref(\"network.proxy.http_port\", already present port)\;', 'user_pref(\"network.proxy.http_port\", your http proxy port);' | Set-Content prefs.js"
5. powershell -Command "(gc prefs.js) -replace 'user_pref(\"network.proxy.share_proxy_settings\", already present value)\;', 'user_pref(\"network.proxy.share_proxy_settings\", true);' | Set-Content prefs.js"

6. powershell -Command "(gc prefs.js) -replace 'user_pref(\"network.proxy.type\", already present value)\;', 'user_pref(\"network.proxy.type\", 1);' | Set-Content prefs.js"

   0 - No Proxy

   1 - Manual Proxy Configuration

   4 - Auto detect Proxy Settings

   5 - Use System Settings(default)

7. cd %windir%

To Check Already Present Proxy Settings

8. cd C:\Users\{username}\AppData\Roaming\Mozilla\Firefox\Profiles\sat2m7dr.default\
9. find /i "network.proxy" prefs.js

To Start Firefox from CMD using your defined proxy settings

12. cd C:\Program Files\
13. cd "Mozilla Firefox"
14. firefox.exe -ProfileManager
15. Select default from the list (default-release is selected by default) and click ok.

NOTE: You may not have to run step 14 again. Instead you can directly run firefox.exe

I don't think there is a direct way to set the proxy (on Windows).

You could however install an add-on like FoxyProxy, create several configurations for different proxies and prior to starting FireFox move the appropriate configuration to the correct folder in your FireFox profile (using a batch file).

Thank very much, I find the answers in this website.

Here I refer to the production of a cmd file

by minimo

cd /D "%APPDATA%\Mozilla\Firefox\Profiles"
cd *.default
set ffile=%cd%
echo user_pref("network.proxy.http", "192.168.1.235 ");>>"%ffile%\prefs.js"
echo user_pref("network.proxy.http_port", 80);>>"%ffile%\prefs.js"
echo user_pref("network.proxy.type", 1);>>"%ffile%\prefs.js"
set ffile=
cd %windir%

user.js is better for customizations as you can include only the lines you want to manipulate, i.e. instead of find-replace you can just overwrite the entire file. Also, prefs.js (at least on Firefox 65.0.1 for Mac) starts with a warning:

// DO NOT EDIT THIS FILE.
//
// If you make changes to this file while the application is running,
// the changes will be overwritten when the application exits.
//
// To change a preference value, you can either:
// - modify it via the UI (e.g. via about:config in the browser); or
// - set it within a user.js file in your profile.

In my case, user.js didn't exist, so I created it and included the line to switch between "No proxy" and "Manual proxy configuration" (I'm using only one SOCKS proxy all the time, so no need to change port number or any other details, just flip 0 to 1 in the following line):

user_pref("network.proxy.type", 1);

I ended up with a bash script that I placed at /usr/local/bin/firefox:

#!/bin/bash
if [ $# -eq 0 ]; then
  echo 'user_pref("network.proxy.type", 0);' > ~/Library/Application\ Support/Firefox/Profiles/t5rvw47o.default/user.js
  open -a Firefox
else
  case $1 in
    vpn)
      echo 'user_pref("network.proxy.type", 1);' > ~/Library/Application\ Support/Firefox/Profiles/t5rvw47o.default/user.js
      open -a Firefox
  esac 
fi

To use it, I make sure no Firefox is running and then run firefox to have a straight connection and firefox vpn to use proxy.

Hello I got the Perfect cod use this code

cd /D "%APPDATA%\Mozilla\Firefox\Profiles"

cd *.default

set ffile=%cd%

echo user_pref("network.proxy.http", "127.0.0.1"); >>prefs.js

echo user_pref("network.proxy.http_port", 8080); >>prefs.js

set ffile=

cd %windir