Hi dudas,

You may find these articles interesting :

https://developer.mozilla.org/fr/docs/Web/HTTP/Gecko_user_agent_string_reference

and :

https://developer.mozilla.org/en-US/docs/Web/HTTP/Browser_detection_using_the_user_agent

and :

https://www.howtogeek.com/113439/how-to-change-your-browsers-user-agent-without-installing-any-extensions/

And finally this ad-on :

https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher-revived/

Hi, you may find what is listed in Google search using your question enough information : https://www.google.com/search?q=How+does+firefox+determine+the+user+agent%3F&ie=utf-8&oe=utf-8&client=firefox-b

https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox

http://www.howtogeek.com/113439/how-to-change-your-browsers-user-agent-without-installing-any-extensions/

Note that it not only the user agent, but there are more navigator properties.

• https://developer.mozilla.org/en-US/docs/Web/API/Navigator

The user agent is send via the HTTP request headers, but JavaScript running on a page can detect more. There are override prefs for most of these, some examples:

• general.useragent.override
• general.platform.override

Note that some properties are spoofed when Resist Fingerprinting is enabled.

• https://dxr.mozilla.org/mozilla-release/source/dom/base/Navigator.cpp