All browsers have user agents and when it operate, it typically identifies itself, its application type, operating system, software vendor, or software revision and each browser has its own data to identify the user agent. But there is no option inside Firefox to switch the user agent. userAgentUpdater is a nice add-on for Firefox which allows you to quickly update the user agent in Firefox right from the context menu. The add-on does not require any restart when installed.

User agent switch

Right click on any area inside Firefox and select the user agent from the option. The add-on has a variety of user agents and supports almost all popular browsers. Supported list includes;

You can download the add-on from here (no restart required).