Bug 1709838: Enable HTTPS-First Mode in PBM Mode in Nightly r=arthuredelstein

Differential Revision: https://phabricator.services.mozilla.com/D114500

modules/libpref/init/StaticPrefList.yaml

@@ -3047,7 +3047,7 @@
 # is set to true then this pref has no effect)
 - name: dom.security.https_first_pbm
   type: RelaxedAtomicBool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
 
 - name: dom.security.unexpected_system_load_telemetry_enabled