Bug 370832 Tab preview doesn't work right in Modern theme

r=neil

themes/modern/navigator/tabbrowser.css

@@ -119,3 +119,7 @@ tab[busy] {
     position: relative;
     list-style-image: url('chrome://global/skin/icons/tab-drag-indicator.gif');
 }
+
+tooltip[tabpreview="true"] {
+  max-width: none;
+}