Extends the functionality of other User-Agent addons, allowing them to modify the User-Agent for JavaScript code.
Source: User-Agent JS Fixer :: Add-ons for Firefox
This Firefox addon changes the Javascript navigator.userAgent.
All it does is to redefine the User-Agent for JavaScript code (navigator.userAgent) using the same string that was sent on the HTTP header.
Use it with addons which changes the HTTP header.
I use this addon with FireMobileSimulator.