In the Firefox search bar, search for about:config and accept the warning message that shows up. Here type general.useragent.override in the preferences search bar. Firefox will offer you to create a new preference with this same name. Select the String option on this and then click on the “+” (plus) icon at the end.

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.