set width to auto for default navbar container, then override it later for fixed navbars (done all via the #gridSystem mixins)

less/mixins.less

@@ -173,8 +173,13 @@
       #gridSystem > .gridColumn(@gridGutterWidth);
     }
     
-    // Fixed container
-    .container { #gridSystem > .columns(@gridGutterWidth, @gridColumnWidth, @gridRowWidth, @gridColumns); }
+    // Fixed containers
+    // Default container and reset for fixed navbars
+    .container,
+    .navbar-fixed-top .container,
+    .navbar-fixed-bottom .container {
+      #gridSystem > .columns(@gridGutterWidth, @gridColumnWidth, @gridRowWidth, @gridColumns);
+    }
 
     // Default columns
     .span1     { #gridSystem > .columns(@gridGutterWidth, @gridColumnWidth, @gridRowWidth, 1); }

less/navbar.less

@@ -20,6 +20,12 @@
   .box-shadow(@shadow);
 }
 
+// Set width to auto for default container
+// We then reset it for fixed navbars in the #gridSystem mixin
+.navbar .container {
+  width: auto;
+}
+
 // Navbar button for toggling navbar items in responsive layouts
 .btn-navbar {
   display: none;