--- a/devtools/client/themes/webconsole.css
+++ b/devtools/client/themes/webconsole.css
@@ -958,22 +958,28 @@ body {
 
 /* Apply a style similar to collapse-button for the object tree arrows */
 .webconsole-app .tree .arrow,
 .webconsole-app .object-inspector .tree-node .arrow {
   width: 10px;
   height: 10px;
   background: url("chrome://devtools/skin/images/arrow.svg") no-repeat center;
   background-size: 10px;
+  transform: rotate(-90deg);
   /* Needed for alignment */
   margin-top: -1px;
   -moz-context-properties: fill;
   fill: var(--theme-icon-dimmed-color);
 }
 
+.webconsole-app .tree .arrow.expanded,
+.webconsole-app .object-inspector .tree-node .arrow.expanded {
+  transform: rotate(0deg);
+}
+
 /* Sidebar */
 .sidebar {
   display: flex;
   grid-row: 1 / -1;
   grid-column: -1 / -2;
   background-color: var(--theme-sidebar-background);
   border-inline-start: 1px solid var(--theme-splitter-color);
 }