Mozilla Firefox
In Mozilla Firefox, this option is buried on Firefox’s about:config page.
To access the about:config page, type about:config into Firefox’s address bar and press Enter. You’ll see a warning–be careful when you change settings here, you could mess up Firefox’s settings.
Type useragent into the filter box. We’re looking for the general.useragent.overridepreference, but it probably won’t exist on your system.
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To create the preference, right-click on the about:config page, point to New, and select String.
Name the preference general.useragent.override .
Enter your desired user agent as the value of the preference. You’ll have to look up your desired user agent on the web and enter it exactly. For example, the following user agent is used by Googlebot, Google’s web crawler:
Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
You can find extensive lists of user agents on various websites, such as this one.
This setting applies to every open tab and persists until you change it, even if you close and reopen Firefox.
To revert Firefox to the default user agent, right-click the “general.useragent.override” preference and select Reset.
