--- a/services/sync/modules/service.js
+++ b/services/sync/modules/service.js
@@ -1745,12 +1745,16 @@ Sync11Service.prototype = {
         this._log.debug("Server returned invalid JSON for '" + info_type +
                         "': " + this.response.body);
         return callback(ex);
       }
       this._log.trace("Successfully retrieved '" + info_type + "'.");
       return callback(null, result);
     });
   },
+
+  recordTelemetryEvent(object, method, value, extra = undefined) {
+    Svc.Obs.notify("weave:telemetry:event", { object, method, value, extra });
+  },
 };
 
 this.Service = new Sync11Service();
 Service.onStartup();

--- a/services/sync/modules/telemetry.js
+++ b/services/sync/modules/telemetry.js
@@ -40,16 +40,18 @@ const TOPICS = [
 
   "weave:engine:sync:start",
   "weave:engine:sync:finish",
   "weave:engine:sync:error",
   "weave:engine:sync:applied",
   "weave:engine:sync:uploaded",
   "weave:engine:validate:finish",
   "weave:engine:validate:error",
+
+  "weave:telemetry:event",
 ];
 
 const PING_FORMAT_VERSION = 1;
 
 // The set of engines we record telemetry for - any other engines are ignored.
 const ENGINES = new Set(["addons", "bookmarks", "clients", "forms", "history",
                          "passwords", "prefs", "tabs", "extension-storage"]);
 
@@ -120,16 +122,54 @@ function tryGetMonotonicTimestamp() {
 function timeDeltaFrom(monotonicStartTime) {
   let now = tryGetMonotonicTimestamp();
   if (monotonicStartTime !== -1 && now !== -1) {
     return Math.round(now - monotonicStartTime);
   }
   return -1;
 }
 
+// This function validates the payload of a telemetry "event" - this can be
+// removed once there are APIs available for the telemetry modules to collect
+// these events (bug XXXXXXX) - but for now we simulate that planned API as best
+// we can.
+function validateTelemetryEvent(eventDetails) {
+  let { object, method, value, extra } = eventDetails;
+  // Do do basic validation of the params - everything except "extra" must
+  // be a string. method and object are required.
+  if (typeof method != "string" || typeof object != "string" ||
+      (value && typeof value != "string") ||
+      (extra && typeof extra != "object")) {
+    log.warn("Invalid event parameters - wrong types", eventDetails);
+    return false;
+  }
+  // length checks.
+  if (method.length > 20 || object.length > 20 ||
+      (value && value.length > 80)) {
+    log.warn("Invalid event parameters - wrong lengths", eventDetails);
+    return false;
+  }
+
+  // extra can be falsey, or an object with string names and values.
+  if (extra) {
+    if (Object.keys(extra).length > 10) {
+      log.warn("Invalid event parameters - too many extra keys", eventDetails);
+      return false;
+    }
+    for (let [ename, evalue] of Object.entries(extra)) {
+      if (typeof ename != "string" || ename.length > 15 ||
+          typeof evalue != "string" || evalue.length > 85) {
+        log.warn(`Invalid event parameters: extra item "${ename} is invalid`, eventDetails);
+        return false;
+      }
+    }
+  }
+  return true;
+}
+
 class EngineRecord {
   constructor(name) {
     // startTime is in ms from process start, but is monotonic (unlike Date.now())
     // so we need to keep both it and when.
     this.startTime = tryGetMonotonicTimestamp();
     this.name = name;
   }
 
@@ -411,36 +451,40 @@ class SyncTelemetryImpl {
     log.level = Log.Level[Svc.Prefs.get("log.logger.telemetry", "Trace")];
     // This is accessible so we can enable custom engines during tests.
     this.allowedEngines = allowedEngines;
     this.current = null;
     this.setupObservers();
 
     this.payloads = [];
     this.discarded = 0;
+    this.events = [];
     this.maxPayloadCount = Svc.Prefs.get("telemetry.maxPayloadCount");
     this.submissionInterval = Svc.Prefs.get("telemetry.submissionInterval") * 1000;
     this.lastSubmissionTime = Telemetry.msSinceProcessStart();
+    this.maxEventsCount = Svc.Prefs.get("telemetry.maxEventsCount", 1000);
   }
 
   getPingJSON(reason) {
     return {
       why: reason,
       discarded: this.discarded || undefined,
       version: PING_FORMAT_VERSION,
       syncs: this.payloads.slice(),
+      events: this.events.length == 0 ? undefined : this.events,
     };
   }
 
   finish(reason) {
     // Note that we might be in the middle of a sync right now, and so we don't
     // want to touch this.current.
     let result = this.getPingJSON(reason);
     this.payloads = [];
     this.discarded = 0;
+    this.events = [];
     this.submit(result);
   }
 
   setupObservers() {
     for (let topic of TOPICS) {
       Observers.add(topic, this, this);
     }
   }
@@ -492,16 +536,49 @@ class SyncTelemetryImpl {
     }
     this.current = null;
     if ((Telemetry.msSinceProcessStart() - this.lastSubmissionTime) > this.submissionInterval) {
       this.finish("schedule");
       this.lastSubmissionTime = Telemetry.msSinceProcessStart();
     }
   }
 
+  _recordEvent(eventDetails) {
+    if (this.events.length >= this.maxEventsCount) {
+      log.warn("discarding event - already queued our maximum", topic);
+      return;
+    }
+
+    if (!validateTelemetryEvent(eventDetails)) {
+      // we've already logged what the problem is...
+      return;
+    }
+    log.debug("recording event", eventDetails);
+
+    let { object, method, value, extra } = eventDetails;
+    let category = "sync";
+    let ts = Math.floor(tryGetMonotonicTimestamp()); // 1-second resolution for privacy
+
+    // An event record is a simple array with at least 4 items.
+    let event = [ts, category, method, object];
+    // It may have up to 6 elements if |extra| is defined
+    if (value) {
+      event.push(value);
+      if (extra) {
+        event.push(extra);
+      }
+    } else {
+      if (extra) {
+        event.push(null); // a null for the empty value.
+        event.push(extra);
+      }
+    }
+    this.events.push(event);
+  }
+
   observe(subject, topic, data) {
     log.trace(`observed ${topic} ${data}`);
 
     switch (topic) {
       case "profile-before-change":
         this.shutdown();
         break;
 
@@ -560,16 +637,20 @@ class SyncTelemetryImpl {
         break;
 
       case "weave:engine:validate:error":
         if (this._checkCurrent(topic)) {
           this.current.onEngineValidateError(data, subject || "Unknown");
         }
         break;
 
+      case "weave:telemetry:event":
+        this._recordEvent(subject);
+        break;
+
       default:
         log.warn(`unexpected observer topic ${topic}`);
         break;
     }
   }
 }
 
 this.SyncTelemetry = new SyncTelemetryImpl(ENGINES);

--- a/services/sync/tests/unit/head_helpers.js
+++ b/services/sync/tests/unit/head_helpers.js
@@ -271,17 +271,25 @@ function get_sync_test_telemetry() {
 
 function assert_valid_ping(record) {
   // This is called as the test harness tears down due to shutdown. This
   // will typically have no recorded syncs, and the validator complains about
   // it. So ignore such records (but only ignore when *both* shutdown and
   // no Syncs - either of them not being true might be an actual problem)
   if (record && (record.why != "shutdown" || record.syncs.length != 0)) {
     if (!SyncPingValidator(record)) {
-      deepEqual([], SyncPingValidator.errors, "Sync telemetry ping validation failed");
+      if (SyncPingValidator.errors.length) {
+        // validation failed - using a simple |deepEqual([], errors)| tends to
+        // truncate the validation errors in the output and doesn't show that
+        // the ping actually was - so be helpful.
+        do_print("telemetry ping validation failed");
+        do_print("the ping data is: " + JSON.stringify(record, undefined, 2));
+        do_print("the validation failures: " + JSON.stringify(SyncPingValidator.errors, undefined, 2));
+        ok(false, "Sync telemetry ping validation failed - see output above for details");
+      }
     }
     equal(record.version, 1);
     record.syncs.forEach(p => {
       lessOrEqual(p.when, Date.now());
       if (p.devices) {
         ok(!p.devices.some(device => device.id == p.deviceID));
         equal(new Set(p.devices.map(device => device.id)).size,
               p.devices.length, "Duplicate device ids in ping devices list");

--- a/services/sync/tests/unit/sync_ping_schema.json
+++ b/services/sync/tests/unit/sync_ping_schema.json
@@ -7,16 +7,21 @@
   "properties": {
     "version": { "type": "integer", "minimum": 0 },
     "discarded": { "type": "integer", "minimum": 1 },
     "why": { "enum": ["shutdown", "schedule"] },
     "syncs": {
       "type": "array",
       "minItems": 1,
       "items": { "$ref": "#/definitions/payload" }
+    },
+    "events": {
+      "type": "array",
+      "minItems": 1,
+      "items": { "$ref": "#/definitions/event" }
     }
   },
   "definitions": {
     "payload": {
       "type": "object",
       "additionalProperties": false,
       "required": ["when", "uid", "took"],
       "properties": {
@@ -123,16 +128,21 @@
         {"required": ["sent"]},
         {"required": ["failed"]}
       ],
       "properties": {
         "sent": { "type": "integer", "minimum": 1 },
         "failed": { "type": "integer", "minimum": 1 }
       }
     },
+    "event": {
+      "type": "array",
+      "minItems": 4,
+      "maxItems": 6
+    },
     "error": {
       "oneOf": [
         { "$ref": "#/definitions/httpError" },
         { "$ref": "#/definitions/nsError" },
         { "$ref": "#/definitions/shutdownError" },
         { "$ref": "#/definitions/authError" },
         { "$ref": "#/definitions/otherError" },
         { "$ref": "#/definitions/unexpectedError" },

--- a/services/sync/tests/unit/test_telemetry.js
+++ b/services/sync/tests/unit/test_telemetry.js
@@ -555,9 +555,97 @@ add_task(async function test_no_foreign_
   await SyncTestingInfrastructure(server);
   try {
     let ping = await sync_and_validate_telem();
     ok(ping.engines.every(e => e.name !== "bogus"));
   } finally {
     Service.engineManager.unregister(engine);
     await cleanAndGo(server);
   }
-});
\ No newline at end of file
+});
+
+add_task(async function test_events() {
+  Service.engineManager.register(BogusEngine);
+  let engine = Service.engineManager.get("bogus");
+  engine.enabled = true;
+  let server = serverForUsers({"foo": "password"}, {
+    meta: {global: {engines: {bogus: {version: engine.version, syncID: engine.syncID}}}},
+    steam: {}
+  });
+
+  await SyncTestingInfrastructure(server);
+  try {
+    Service.recordTelemetryEvent("object", "method", "value", { foo: "bar" });
+    let ping = await wait_for_ping(() => Service.sync(), true, true);
+    equal(ping.events.length, 1);
+    let [timestamp, category, method, object, value, extra] = ping.events[0];
+    ok((typeof timestamp == "number") && timestamp > 0); // timestamp.
+    equal(category, "sync");
+    equal(method, "method");
+    equal(object, "object");
+    equal(value, "value");
+    deepEqual(extra, { foo: "bar" });
+    // Test with optional values.
+    Service.recordTelemetryEvent("object", "method");
+    ping = await wait_for_ping(() => Service.sync(), false, true);
+    equal(ping.events.length, 1);
+    equal(ping.events[0].length, 4);
+
+    Service.recordTelemetryEvent("object", "method", "extra");
+    ping = await wait_for_ping(() => Service.sync(), false, true);
+    equal(ping.events.length, 1);
+    equal(ping.events[0].length, 5);
+
+    Service.recordTelemetryEvent("object", "method", undefined, { foo: "bar" });
+    ping = await wait_for_ping(() => Service.sync(), false, true);
+    equal(ping.events.length, 1);
+    equal(ping.events[0].length, 6);
+    [timestamp, category, method, object, value, extra] = ping.events[0];
+    equal(value, null);
+  } finally {
+    Service.engineManager.unregister(engine);
+    await cleanAndGo(server);
+  }
+});
+
+add_task(async function test_invalid_events() {
+  Service.engineManager.register(BogusEngine);
+  let engine = Service.engineManager.get("bogus");
+  engine.enabled = true;
+  let server = serverForUsers({"foo": "password"}, {
+    meta: {global: {engines: {bogus: {version: engine.version, syncID: engine.syncID}}}},
+    steam: {}
+  });
+
+  async function checkNotRecorded(...args) {
+    Service.recordTelemetryEvent.call(args);
+    let ping = await wait_for_ping(() => Service.sync(), false, true);
+    equal(ping.events, undefined);
+  }
+
+  await SyncTestingInfrastructure(server);
+  try {
+    let long21 = "l".repeat(21);
+    let long81 = "l".repeat(81);
+    let long86 = "l".repeat(86);
+    await checkNotRecorded("object");
+    await checkNotRecorded("object", 2);
+    await checkNotRecorded(2, "method");
+    await checkNotRecorded("object", "method", 2);
+    await checkNotRecorded("object", "method", "value", 2);
+    await checkNotRecorded("object", "method", "value", { foo: 2 });
+    await checkNotRecorded(long21, "method", "value");
+    await checkNotRecorded("object", long21, "value");
+    await checkNotRecorded("object", "method", long81);
+    let badextra = {};
+    badextra[long21] = "x";
+    await checkNotRecorded("object", "method", "value", badextra);
+    badextra = { "x": long86 };
+    await checkNotRecorded("object", "method", "value", badextra);
+    for (let i = 0; i < 10; i++) {
+      badextra["name" + i] = "x";
+    }
+    await checkNotRecorded("object", "method", "value", badextra);
+  } finally {
+    Service.engineManager.unregister(engine);
+    await cleanAndGo(server);
+  }
+});