--- a/toolkit/components/telemetry/TelemetryEvent.cpp
+++ b/toolkit/components/telemetry/TelemetryEvent.cpp
@@ -541,20 +541,24 @@ RegisterEvents(const StaticMutexAutoLock
   // Register the new events.
   if (!gDynamicEventInfo) {
     gDynamicEventInfo = new nsTArray<DynamicEventInfo>();
   }
 
   for (uint32_t i = 0, len = eventInfos.Length(); i < len; ++i) {
     const nsCString& eventName = UniqueEventName(eventInfos[i]);
 
-    // Re-registering events can happen when add-ons update, so we don't print warnings.
-    // We don't support changing their definition, but the expiry might have changed.
+    // Re-registering events can happen for two reasons and we don't print warnings:
+    //
+    // * When add-ons update.
+    //   We don't support changing their definition, but the expiry might have changed.
+    // * When dynamic builtins ("build faster") events are registered.
+    //   The dynamic definition takes precedence then.
     EventKey* existing = nullptr;
-    if (gEventNameIDMap.Get(eventName, &existing)) {
+    if (!aBuiltin && gEventNameIDMap.Get(eventName, &existing)) {
       if (eventExpired[i]) {
         existing->id = kExpiredEventId;
       }
       continue;
     }
 
     gDynamicEventInfo->AppendElement(eventInfos[i]);
     uint32_t eventId = eventExpired[i] ? kExpiredEventId : gDynamicEventInfo->Length() - 1;

--- a/toolkit/components/telemetry/tests/unit/test_TelemetryEvents_buildFaster.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetryEvents_buildFaster.js
@@ -232,8 +232,110 @@ add_task(async function test_dynamicBuil
   ];
   let events = snapshot.parent;
   Assert.equal(events.length, expected.length, "Should have recorded the right amount of events.");
   for (let i = 0; i < expected.length; ++i) {
     Assert.deepEqual(events[i].slice(1), expected[i],
                      "Should have recorded the expected event data.");
   }
 });
+
+add_task(async function test_dynamicBuiltinEventsOverridingStatic() {
+  Telemetry.clearEvents();
+  Telemetry.canRecordExtended = true;
+
+  const TEST_EVENT_NAME = "telemetry.test";
+
+  // Register dynamic builtin test events, overwriting existing one.
+  Telemetry.registerBuiltinEvents(TEST_EVENT_NAME, {
+    // Event with only required fields.
+    "test1": {
+      methods: ["test1"],
+      objects: ["object1", "object2"],
+    },
+    // Event with extra_keys.
+    "test2": {
+      methods: ["test2"],
+      objects: ["object1", "object2", "object3"],
+      extra_keys: ["key1", "key2", "newdynamickey"],
+    },
+  });
+
+  // Record some events that should be available in the static event already .
+  Telemetry.setEventRecordingEnabled(TEST_EVENT_NAME, true);
+  Telemetry.recordEvent(TEST_EVENT_NAME, "test1", "object1");
+  Telemetry.recordEvent(TEST_EVENT_NAME, "test2", "object1", null,
+                        {"key1": "foo", "key2": "bar"});
+  // Record events with newly added objects and keys.
+  Telemetry.recordEvent(TEST_EVENT_NAME, "test2", "object2", null,
+                        {"newdynamickey": "foo"});
+  Telemetry.recordEvent(TEST_EVENT_NAME, "test2", "object3", null,
+                        {"key1": "foo"});
+  // Now check that the snapshot contains the expected data.
+  let snapshot =
+    Telemetry.snapshotEvents(Ci.nsITelemetry.DATASET_RELEASE_CHANNEL_OPTIN, false);
+  Assert.ok(("parent" in snapshot), "Should have parent events in the snapshot.");
+
+  let expected = [
+    [TEST_EVENT_NAME, "test1", "object1"],
+    [TEST_EVENT_NAME, "test2", "object1", null, {key1: "foo", key2: "bar"}],
+    [TEST_EVENT_NAME, "test2", "object2", null, {newdynamickey: "foo"}],
+    [TEST_EVENT_NAME, "test2", "object3", null, {key1: "foo"}],
+  ];
+  let events = snapshot.parent;
+  Assert.equal(events.length, expected.length, "Should have recorded the right amount of events.");
+  for (let i = 0; i < expected.length; ++i) {
+    Assert.deepEqual(events[i].slice(1), expected[i],
+                     "Should have recorded the expected event data.");
+  }
+});
+
+add_task(async function test_realDynamicDontOverwrite() {
+  // Real dynamic events follow similar code paths internally.
+  // Let's ensure they trigger the right code path and don't overwrite.
+
+  Telemetry.clearEvents();
+  Telemetry.canRecordExtended = true;
+
+  const TEST_EVENT_NAME = "telemetry.test";
+
+  // Register dynamic test events, this should not overwrite existing ones.
+  Telemetry.registerEvents(TEST_EVENT_NAME, {
+    // Event with only required fields.
+    "test1": {
+      methods: ["test1"],
+      objects: ["object1", "object2"],
+    },
+    // Event with extra_keys.
+    "test2": {
+      methods: ["test2"],
+      objects: ["object1", "object2", "object3"],
+      extra_keys: ["key1", "key2", "realdynamic"],
+    },
+  });
+
+  // Record some events that should be available in the static event already .
+  Telemetry.setEventRecordingEnabled(TEST_EVENT_NAME, true);
+  Telemetry.recordEvent(TEST_EVENT_NAME, "test1", "object1");
+  Telemetry.recordEvent(TEST_EVENT_NAME, "test2", "object1", null,
+                        {"key1": "foo", "key2": "bar"});
+  // Record events with newly added objects and keys.
+  Telemetry.recordEvent(TEST_EVENT_NAME, "test2", "object2", null,
+                        {"realdynamic": "foo"});
+  Telemetry.recordEvent(TEST_EVENT_NAME, "test2", "object3", null,
+                        {"key1": "foo"});
+  // Now check that the snapshot contains the expected data.
+  let snapshot =
+    Telemetry.snapshotEvents(Ci.nsITelemetry.DATASET_RELEASE_CHANNEL_OPTIN, false);
+  Assert.ok(("parent" in snapshot), "Should have parent events in the snapshot.");
+
+  let expected = [
+    [TEST_EVENT_NAME, "test1", "object1"],
+    [TEST_EVENT_NAME, "test2", "object1", null, {key1: "foo", key2: "bar"}],
+    [TEST_EVENT_NAME, "test2", "object3", null, {key1: "foo"}],
+  ];
+  let events = snapshot.parent;
+  Assert.equal(events.length, expected.length, "Should have recorded the right amount of events.");
+  for (let i = 0; i < expected.length; ++i) {
+    Assert.deepEqual(events[i].slice(1), expected[i],
+                     "Should have recorded the expected event data.");
+  }
+});