gh-13853: Fixed sidebar flickering on private windows (gh-13855)

prefs/firefox/browser.yaml

@@ -87,6 +87,10 @@
   value: false
   locked: true
 
+# See gh-13853 for details on why we should disable this preference
+- name: browser.privatebrowsing.resetPBM.enabled
+  value: false
+
 # See gh-12985 for details on the following preferences
 - name: browser.search.widget.new
   value: true