Launch Firefox with a Separate SOCKS Proxy Test Profile

A separate Firefox profile can carry a repeatable manual proxy setup. Prepare a dedicated profile directory and, while that profile is closed, place these values in its user.js:

user_pref("network.proxy.type", 1);
user_pref("network.proxy.socks", "proxy.example");
user_pref("network.proxy.socks_port", 1080);
user_pref("network.proxy.socks_version", 5);

For a Linux launch using the prepared directory:

firefox --new-instance --profile "/home/user/firefox-proxy-test"

Replace the directory and endpoint; the SOCKS5 proxy list offers candidates for a permitted public-site test. --profile selects the intended profile, while --new-instance requests a separate process rather than sending the launch to an already running instance. The same profile must not be open in another Firefox process.

user.js supplies preferences when the profile loads. Editing it is not a live proxy-switching API. Firefox also writes saved user preferences to prefs.js, so deleting user.js alone may leave an already saved value in place; reset the relevant saved preferences when retiring the test configuration.

This setup belongs to that profile. It does not reconfigure the operating system or another Firefox profile. Managed policy can also constrain which preferences are effective.