User Agent Overrider is a Firefox extension meant to override the user-agent string.
Hosted at AMO, fully reviewed by AMO editors.
https://addons.mozilla.org/firefox/addon/user-agent-overrider/
Also hosted at AMO, development channel
https://addons.mozilla.org/firefox/addon/user-agent-overrider/versions/
The contents of this repository are the same as the contents of User Agent Overrider's xpi file.
So just checkout/download the code and archive all the files (not the toplevel folder) to a zip file.
Then rename the filename suffix from ".zip" to ".xpi" and install the extension using the xpi file.
The translation files are hosted at Transifex.
If you want to help translating this project into your native language, take a look at the given page
https://www.transifex.com/projects/p/user_agent_overrider/