1. Open Firefox browser.
2. In the address bar type about:config.
3. Warning may appear, just click I'll be careful, I promise.
4. Right-click point to New>String.
5. Type your user-agent in the textbox and click OK
Here you can find lists of user-agent:
CHANGING CHROME USER-AGENT
1. Find the shortcut of Google Chrome.
2. Right-click and choose Properties
3. In the target area Add this string --user-agent="Your user-agent string" and click Apply
4. You're Done.