There is a few easy steps that you need to do to change your user agent. This method might seem difficult but this is the native way to change the user agent without the assistance of an extension. There is also a video tutorial below that should aid you in following these steps.
StepsThe first thing you need to do is open a new Firefox window or tab. In the URL box, type in "about:config". This is your browser's settings, in a more raw form. You will get a warning telling you that this "might void your warrenty". All this means is that changing these settings the wrong way can mess up your browser.
In the search bar on the page, type in "useragent". Check to see if there is a user agent called "general.useragent.override". If there is, then you can just modify the value of the string. If that exact phrase I just listed is not there, then you need to create a new sting. To do that, right click somewhere in the white space, hover over "New", and then select "String". When a box appears, enter the part in bold, and then the value is your new user agent.
In the image below, ignore the highlighted option. This is as if you don't have this specific entry in the config:
The next box you will see, the value, is the user agent that you wish to use with Firefox. You can copy a user agent from a website that lists popular user agents, or you can create your own if you want. The user agent I'm putting into this box you're about to see is Mozilla/5.0 (Linux; U; Android 2.3.4; en-us; LGL75C Build/GRJ22) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1.
After that your user agent setting should be bold in the about:config window. The user agent takes affect as soon as you set it. If you wish to reset Firefox back to it's default, native user agent, just right click on the value, and choose reset. The value field will be cleared.
The string goes on longer than that, but that is how the listing should look.