firefox: move tab bar to the right by default

conf/firefox-forbidden-policies.js

@@ -86,6 +86,7 @@ lockPref("toolkit.telemetry.dap_enabled", true);
 // New sidebar
 lockPref("sidebar.revamp", true);
 lockPref("sidebar.verticalTabs", true);
+lockPref("sidebar.position_start", false);
 lockPref("sidebar.visibility", "always-show");
 
 // Per process isolation

etc/firefox/policies/policies.json

@@ -1192,6 +1192,12 @@
 				"Type": "boolean",
 				"Value": true
 			},
+			"sidebar.position_start": {
+				"Comment": "Tab bar on the right (what a logical name?). Preference not allowed for stability reasons :(",
+				"Status": "default",
+				"Type": "boolean",
+				"Value": false
+			},
 			"sidebar.revamp": {
 				"Comment": "Preference not allowed for stability reasons.",
 				"Status": "default",