Use a pref for supported Sync server API levels

application/palemoon/branding/shared/pref/preferences.inc

@@ -115,6 +115,7 @@ pref("image.mem.decode_bytes_at_a_time", 65536); //larger chunks
 // Sync server URL
 pref("services.sync.serverURL","https://pmsync.palemoon.org/sync/index.php/");
 pref("services.sync.jpake.serverURL","https://keyserver.palemoon.org/");
+pref("services.sync.APILevel", 1); // FSyncMS doesn't support info/configuration
 
 // ============================================================================
 

services/sync/modules/service.js

@@ -1067,8 +1067,9 @@ Sync11Service.prototype = {
   // Note: returns false if we failed for a reason other than the server not yet
   // supporting the api.
   _fetchServerConfiguration() {
-#if 0 // FSyncMS doesn't support this and it will break auth if left enabled!
+    if (Svc.Prefs.get("APILevel") >= 2) {
       // This is similar to _fetchInfo, but with different error handling.
+      // Only supported by later sync implementations.
 
       let infoURL = this.userBaseURL + "info/configuration";
       this._log.debug("Fetching server configuration", infoURL);
@@ -1093,7 +1094,7 @@ Sync11Service.prototype = {
         this.serverConfiguration = configResponse.obj;
       }
       this._log.trace("info/configuration for this server", this.serverConfiguration);
-#endif
+    }
     return true;
   },
 

services/sync/services-sync.js

@@ -12,6 +12,7 @@ pref("services.sync.syncKeyHelpURL", "https://services.mozilla.com/help/synckey"
 
 pref("services.sync.lastversion", "firstrun");
 pref("services.sync.sendVersionInfo", true);
+pref("services.sync.APILevel", 2);
 
 pref("services.sync.scheduler.eolInterval",         604800); // 1 week
 pref("services.sync.scheduler.idleInterval",         3600);  // 1 hour