Turning privacy.resistFingerprinting already changes your user agent to the latest ESR release (v52). Are you trying to change it to something else? Use an extension like User Agent Switcher revived.

The general.useragent.override pref doesn't exist by default, so you would have to create a a String pref to use this feature.

I tested it in the current beta and with privacy.resistFingerprinting = true the user agent is not modified with general.useragent.override, so privacy.resistFingerprinting seems to be applied at a later stage and overrides other modifications.

using a User Agent Switcher did the trick