Which preference sets OS and version?
I would like to be able to control which OS and version is reported by my browser to sites like https://www.whatsmybrowser.org/ . I can do about:config but I don't see which setting has that info, to change it. I also see a User Agent Switcher extension, but it doesn't let you set the version of the OS. https://www.whatsmybrowser.org/ clearly shows my OS and version, so I know it's there somewhere. How do I change it?
Opaite Mbohovái (2)
You actually need to create one or two user agent preferences from scratch. I can't tell you what would work best for any given site, but using older values can cause sites to declare that your browser is no longer supported, so some experimentation might be needed.
Here's an example thread: https://support.mozilla.org/en-US/questions/1365101#answer-1476335
Also, is this the add-on you looked at: https://addons.mozilla.org/firefox/addon/user-agent-string-switcher/
Note that there are more override prefs available.
general.oscpu.override general.buildID.override general.platform.override general.appversion.override general.appname.override general.useragent.override