new file mode 100644
--- /dev/null
+++ b/js/xpconnect/tests/unit/test_js_memory_telemetry.js
@@ -0,0 +1,55 @@
+"use strict";
+
+const {Services} = ChromeUtils.import("resource://gre/modules/Services.jsm", {});
+
+add_task(function test_compartment_realm_counts() {
+  const compsSystem = "MEMORY_JS_COMPARTMENTS_SYSTEM";
+  const compsUser = "MEMORY_JS_COMPARTMENTS_USER";
+  const realmsSystem = "MEMORY_JS_REALMS_SYSTEM";
+  const realmsUser = "MEMORY_JS_REALMS_USER";
+
+  Cu.forceShrinkingGC();
+
+  Services.telemetry.gatherMemory();
+  let snapshot1 = Services.telemetry.getSnapshotForHistograms("main", true).parent;
+
+  // We can't hard code exact counts, but we can check some basic invariants:
+  //
+  // * Compartments must contain at least one realm, so there must be more
+  //   realms than compartments.
+  // * There must be at least one system realm.
+
+  Assert.ok(snapshot1[realmsSystem].sum <= snapshot1[compsSystem].sum,
+            "Number of system compartments can't exceed number of system realms");
+  Assert.ok(snapshot1[realmsUser].sum <= snapshot1[compsUser].sum,
+            "Number of user compartments can't exceed number of user realms");
+  Assert.ok(snapshot1[realmsSystem].sum > 0,
+            "There must be at least one system realm");
+
+  // Now we create a bunch of sandboxes (more than one to be more resilient
+  // against GCs happening in the meantime), so we can check:
+  //
+  // * There are now more realms and user compartments than before. Not system
+  //   compartments, because system realms share a compartment.
+  // * The system compartment contains multiple realms.
+
+  let systemPrincipal = Services.scriptSecurityManager.getSystemPrincipal();
+  let arr = [];
+  for (let i = 0; i < 5; i++) {
+    arr.push(Cu.Sandbox(null));
+    arr.push(Cu.Sandbox(systemPrincipal));
+  }
+
+  Services.telemetry.gatherMemory();
+  let snapshot2 = Services.telemetry.getSnapshotForHistograms("main", true).parent;
+
+  for (let k of [realmsSystem, realmsUser, compsUser]) {
+    Assert.ok(snapshot2[k].sum > snapshot1[k].sum,
+              "There must be more compartments/realms now: " + k);
+  }
+
+  Assert.ok(snapshot2[realmsSystem].sum > snapshot2[compsSystem].sum,
+            "There must be more system realms than system compartments now");
+
+  arr[0].x = 10; // Ensure the JS engine keeps |arr| alive until this point.
+});

--- a/js/xpconnect/tests/unit/xpcshell.ini
+++ b/js/xpconnect/tests/unit/xpcshell.ini
@@ -115,16 +115,17 @@ head = head_ongc.js
 [test_URLSearchParams.js]
 [test_fileReader.js]
 [test_messageChannel.js]
 [test_crypto.js]
 [test_css.js]
 [test_rtcIdentityProvider.js]
 [test_sandbox_atob.js]
 [test_isProxy.js]
+[test_js_memory_telemetry.js]
 [test_getObjectPrincipal.js]
 [test_sandbox_name.js]
 [test_watchdog_enable.js]
 head = head_watchdog.js
 [test_watchdog_disable.js]
 head = head_watchdog.js
 [test_watchdog_toggle.js]
 head = head_watchdog.js

--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -1103,16 +1103,36 @@
     "record_in_processes": ["main", "content"],
     "alert_emails": ["memshrink-telemetry-alerts@mozilla.com"],
     "expires_in_version": "never",
     "kind": "exponential",
     "high": 1000,
     "n_buckets": 50,
     "description": "Total JavaScript compartments used for web pages"
   },
+  "MEMORY_JS_REALMS_SYSTEM": {
+    "record_in_processes": ["main", "content"],
+    "alert_emails": ["memshrink-telemetry-alerts@mozilla.com", "jdemooij@mozilla.com"],
+    "bug_numbers": [1518077],
+    "expires_in_version": "never",
+    "kind": "exponential",
+    "high": 1000,
+    "n_buckets": 50,
+    "description": "Total JavaScript realms used for add-ons and internals."
+  },
+  "MEMORY_JS_REALMS_USER": {
+    "record_in_processes": ["main", "content"],
+    "alert_emails": ["memshrink-telemetry-alerts@mozilla.com", "jdemooij@mozilla.com"],
+    "bug_numbers": [1518077],
+    "expires_in_version": "never",
+    "kind": "exponential",
+    "high": 1000,
+    "n_buckets": 50,
+    "description": "Total JavaScript realms used for web pages."
+  },
   "MEMORY_JS_GC_HEAP": {
     "record_in_processes": ["main", "content"],
     "alert_emails": ["memshrink-telemetry-alerts@mozilla.com"],
     "expires_in_version": "never",
     "kind": "exponential",
     "low": 1024,
     "high": 16777216,
     "n_buckets": 200,

--- a/xpcom/base/MemoryTelemetry.cpp
+++ b/xpcom/base/MemoryTelemetry.cpp
@@ -268,16 +268,18 @@ nsresult MemoryTelemetry::GatherReports(
 
   // Collect cheap or main-thread only metrics synchronously, on the main
   // thread.
   RECORD(MEMORY_JS_GC_HEAP, JSMainRuntimeGCHeap, UNITS_BYTES);
   RECORD(MEMORY_JS_COMPARTMENTS_SYSTEM, JSMainRuntimeCompartmentsSystem,
          UNITS_COUNT);
   RECORD(MEMORY_JS_COMPARTMENTS_USER, JSMainRuntimeCompartmentsUser,
          UNITS_COUNT);
+  RECORD(MEMORY_JS_REALMS_SYSTEM, JSMainRuntimeRealmsSystem, UNITS_COUNT);
+  RECORD(MEMORY_JS_REALMS_USER, JSMainRuntimeRealmsUser, UNITS_COUNT);
   RECORD(MEMORY_IMAGES_CONTENT_USED_UNCOMPRESSED, ImagesContentUsedUncompressed,
          UNITS_BYTES);
   RECORD(MEMORY_STORAGE_SQLITE, StorageSQLite, UNITS_BYTES);
 #ifdef XP_WIN
   RECORD(LOW_MEMORY_EVENTS_VIRTUAL, LowMemoryEventsVirtual,
          UNITS_COUNT_CUMULATIVE);
   RECORD(LOW_MEMORY_EVENTS_COMMIT_SPACE, LowMemoryEventsCommitSpace,
          UNITS_COUNT_CUMULATIVE);