--- a/browser/components/nsBrowserGlue.js
+++ b/browser/components/nsBrowserGlue.js
@@ -1682,17 +1682,17 @@ BrowserGlue.prototype = {
         return;
       this._openPreferences("sync", { origin: "doorhanger" });
     }
     this.AlertsService.showAlertNotification(null, title, body, true, null, clickCallback);
   },
 
   // eslint-disable-next-line complexity
   _migrateUI: function BG__migrateUI() {
-    const UI_VERSION = 55;
+    const UI_VERSION = 56;
     const BROWSER_DOCURL = "chrome://browser/content/browser.xul";
 
     let currentUIVersion;
     if (Services.prefs.prefHasUserValue("browser.migration.version")) {
       currentUIVersion = Services.prefs.getIntPref("browser.migration.version");
     } else {
       // This is a new profile, nothing to migrate.
       Services.prefs.setIntPref("browser.migration.version", UI_VERSION);
@@ -2012,24 +2012,16 @@ BrowserGlue.prototype = {
           // In this case just fallback to the safest side and disable suggestions.
           Services.prefs.setBoolPref("browser.urlbar.suggest.searches", false);
         }
       } catch (ex) {
         // A missing pref is not a fatal error.
       }
     }
 
-    if (currentUIVersion < 48) {
-      // Bug 1372954 - the checked value was persisted but the attribute removal wouldn't
-      // be persisted (Bug 15232). Turns out we can just not persist the value in this case.
-      // The situation was only happening for a few nightlies in 56, so this migration can
-      // be removed in version 58.
-      xulStore.removeValue(BROWSER_DOCURL, "sidebar-box", "checked");
-    }
-
     if (currentUIVersion < 49) {
       // Annotate that a user haven't seen any onboarding tour
       Services.prefs.setIntPref("browser.onboarding.seen-tourset-version", 0);
     }
 
     if (currentUIVersion < 50) {
       try {
         // Transform prefs related to old DevTools Console.
@@ -2086,16 +2078,27 @@ BrowserGlue.prototype = {
         Services.prefs.clearUserPref("browser.onboarding.hidden");
       }
     }
 
     if (currentUIVersion < 55) {
       Services.prefs.clearUserPref("browser.customizemode.tip0.shown");
     }
 
+    if (currentUIVersion < 56) {
+      // Prior to the end of the Firefox 57 cycle, the sidebarcommand being present
+      // or not was the only thing that distinguished whether the sidebar was open.
+      // Now, the sidebarcommand always indicates the last opened sidebar, and we
+      // correctly persist the checked attribute to indicate whether or not the
+      // sidebar was open. We should set the checked attribute in case it wasn't:
+      if (xulStore.getValue(BROWSER_DOCURL, "sidebar-box", "sidebarcommand")) {
+        xulStore.setValue(BROWSER_DOCURL, "sidebar-box", "checked", "true");
+      }
+    }
+
     // Update the migration version.
     Services.prefs.setIntPref("browser.migration.version", UI_VERSION);
   },
 
   _checkForDefaultBrowser() {
     // Perform default browser checking.
     if (!ShellService) {
       return;