https://www.whatismybrowser.com/guides/how-to-change-your-user-agent/firefox
Changing your user agent in Firefox takes a few simple steps.
Open a new tab
While Firefox has a standard settings interface for changing common settings, the interface for changing advanced settings such as the user agent in Firefox is done via the browser itself. Please open a blank window or tab.
Browse to "about:config"
In the address bar, type in about:config and press the "Go" arrow
Dismiss the warning
The "about:config" system will give you access to change special settings which might make your Firefox unusable or behave strangely. Firefox will give you a warning about this to make you aware of it.
As long as you follow the instructions here, you should be fine.
Create a new preference item
Firefox's settings are stored as a series of Preference Names and Values. The initial display of "about:config" will show you the hundreds of settings which Firefox is using. You can scroll through them to have a look if you're curious. Just be careful to not change anything unintentionally!
The setting to change your user agent doesn't exist in a default installation of Firefox; you need to create it yourself.
To do that, you can Right-Click any where on the list of preferences and go to the "New" item which will be in the menu that appears. The "New" menu item then expands to show you three options: "String", "Integer" and "Boolean".
Create a new "String" preference by clicking on it.
Firefox will ask you for the new Preference name: enter general.useragent.override and Press "OK"
You are then prompted for the user agent value you want to use. You can paste or type in whatever user agent you need to use. (If you're just curious to try out user agents, you can browse our listing of user agents to find different ones to try out!)
Once you've entered your desired user agent string, press "OK" again. Firefox will now use that user agent when you make requests.
You can test out your new user agent by visiting our homepage and setting what we detect it as!
Close the Preferences window
Now that you have selected your default browser for macOS, you can close the Preferences window (by clicking the red dot in the top-left corner) and resume your web browsing.
Resetting your user agent in Firefox
Firefox will continue to use the user agent you've set until you delete the preference you've created.
To do that, search for the general.useragent.override preference in the "about:config" window; if you've still got the preference in your system, it will appear, displaying what you've changed it to. If it doesn't appear then Firefox presumably isn't overriding your user agent any more. Double check what Firefox is sending as your user agent. Maybe the setting has already been reset?
Once you have found the preference that is overriding your user agent, get rid of it by Right-Clicking on it and selecting "Reset".
The preference item will still stay there but it will no longer be bolded, the value column will be emptied. Your user agent is now reset back to it's default firefox user agent. The next time you start Firefox it will disappear completely.
You have now changed (or reset!) your user agent string in Firefox. We hope this guide was useful.
If you need to parse user agents in your system, please check out our User Agent Parsing API. It will make your life much easier!