if the key doesn't exist yet, select the String option and click + button
enter the useragent, eg. Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/127.0.0.0 Safari/537.36 for Chrome/Windows
Don't do this. It will override the user agent for every site you visit which will cause a lot of problems, like failing most captchas and breaking a lot of websites.
You should use an extension (like this one) to change UA on a per site basis instead.
Many people have had easy success with Firefox by spoofing a “Chrome on Windows” user agent; you can use an extension like this one to do so.
about:config
general.useragent.override
String
option and click+
buttonMozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/127.0.0.0 Safari/537.36
for Chrome/WindowsDon't do this. It will override the user agent for every site you visit which will cause a lot of problems, like failing most captchas and breaking a lot of websites.
You should use an extension (like this one) to change UA on a per site basis instead.
Or do the sane thing: close tab on any site that refuses to support Fx