Clarify yourself. It is all about what extension do you use?
I would use Google Chrome as web-browser, and “User-Agent Switcher for Chrome” extension.
Then navigate to Extensions page, click on “Options” link under the name of User-Agent Switcher for Chrome extension.
This will bring up new web-page for managing user-agents. It is very easy to create one from there.
Just fill in first two fields with name of the agent.. Use “Replace”.. Indicator, that’s what text will be shown on the icon in browser-extension section.. I usually just put 1 there.. Once you add it, to activate it, close Extensions tab, go to another website, or just open new tab, then click on user-agent switcher extension icon in browser-extension section (top-right corner), then click the name, and then the name again..
-or-
Go look at FAQ of the extension you have downloaded for Mozilla Firefox, you should find the answer there.
P.S. To verify, you can visit here.