A user agent is a line of code that tells websites what type of browser and operating system you're using. This information helps the web server deliver content that's compatible with your browser.
You can change Firefox's user agent to trick outdated websites into thinking that you're using a different version of the browser. Web developers might do this to see how their websites will appear on different versions of a browser. Sometimes, add-ons and other installations can change your user agent without your knowledge. In any case, it's important to switch it back to the default user agent to display most websites correctly.
To reset your user agent:
In the address bar, type about:config and press EnterReturn.
The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise!I accept the risk! to continue to the about:config page.
Search for useragent by typing it into the FilterSearch field.
Right-clickHold down the Ctrl key while you click
on each of these preferences, if they exist, and select Reset from the context menu.
general.useragent.extra.productName - where productName can be the name of some third party software (e.g. microsoftdotnet).
After resetting your user agent, you may need to clear your cookies and cache:
Click the menu button
, choose History and select Clear Recent History....
In the Time Range to clear: drop-down, select Everything.
Click the arrow next to Details to display the list of items that can be cleared.