--- a/devtools/client/inspector/inspector.js
+++ b/devtools/client/inspector/inspector.js
@@ -52,16 +52,17 @@ const INITIAL_SIDEBAR_SIZE = 350;
 const PORTRAIT_MODE_WIDTH_THRESHOLD = 700;
 // If the toolbox's width docked to the side is smaller than the given amount of pixels,
 // the sidebar automatically switches from 'landscape/horizontal' to 'portrait/vertical'
 // mode.
 const SIDE_PORTAIT_MODE_WIDTH_THRESHOLD = 1000;
 
 const SHOW_THREE_PANE_TOGGLE_PREF = "devtools.inspector.three-pane-toggle";
 const THREE_PANE_ENABLED_PREF = "devtools.inspector.three-pane-enabled";
+const THREE_PANE_ENABLED_SCALAR = "devtools.inspector.three_pane_enabled";
 
 /**
  * Represents an open instance of the Inspector for a tab.
  * The inspector controls the breadcrumbs, the markup view, and the sidebar
  * (computed view, rule view, font view and animation inspector).
  *
  * Events:
  * - ready
@@ -286,18 +287,24 @@ Inspector.prototype = {
     // All the components are initialized. Let's select a node.
     if (defaultSelection) {
       let onAllPanelsUpdated = this.once("inspector-updated");
       this.selection.setNodeFront(defaultSelection, { reason: "inspector-open" });
       await onAllPanelsUpdated;
       await this.markup.expandNode(this.selection.nodeFront);
     }
 
-    // And setup the toolbar only now because it may depend on the document.
+    // Setup the toolbar only now because it may depend on the document.
     await this.setupToolbar();
+
+    // Log the 3 pane inspector setting on inspector open. The question we want to answer
+    // is:
+    // "What proportion of users use the 3 pane vs 2 pane inspector on inspector open?"
+    this.telemetry.logKeyedScalar(THREE_PANE_ENABLED_SCALAR, this.is3PaneModeEnabled, 1);
+
     this.emit("ready");
     return this;
   },
 
   _onBeforeNavigate: function() {
     this._defaultNode = null;
     this.selection.setNodeFront(null);
     this._destroyMarkup();

--- a/toolkit/components/telemetry/Scalars.yaml
+++ b/toolkit/components/telemetry/Scalars.yaml
@@ -878,16 +878,33 @@ devtools.grid.showInfiniteLines:
     expires: never
     kind: uint
     notification_emails:
       - dev-developer-tools@lists.mozilla.org
     release_channel_collection: opt-out
     record_in_processes:
       - 'main'
 
+devtools.inspector:
+  three_pane_enabled:
+    bug_numbers:
+      - 1437881
+    description: >
+      Number of times the DevTools inspector was opened with the 3 pane inspector enabled,
+      keyed by true/false.
+    expires: "65"
+    kind: uint
+    keyed: true
+    notification_emails:
+      - dev-developer-tools@lists.mozilla.org
+      - gl@mozilla.com
+    release_channel_collection: opt-out
+    record_in_processes:
+      - 'main'
+
 devtools:
   current_theme:
     bug_numbers:
       - 1396811
     description: >
       Number of times DevTools was opened, keyed by theme.
     expires: never
     kind: uint