Bug 1761690 - Let overlay scrollbars on GTK ride the trains. r=stransky

Differential Revision: https://phabricator.services.mozilla.com/D142173

modules/libpref/init/StaticPrefList.yaml

@@ -12941,7 +12941,7 @@
 # Whether we use overlay scrollbars on GTK.
 - name: widget.gtk.overlay-scrollbars.enabled
   type: RelaxedAtomicBool
-  value: @IS_EARLY_BETA_OR_EARLIER@
+  value: true
   mirror: always
 
 # Whether we honor the scrollbar colors from the gtk theme.