diff --git a/toolkit/components/telemetry/TelemetryHistograms.h b/toolkit/components/telemetry/TelemetryHistograms.h
index ff82f8c6b2bf..3b025984ae09 100644
--- a/toolkit/components/telemetry/TelemetryHistograms.h
+++ b/toolkit/components/telemetry/TelemetryHistograms.h
@@ -414,6 +414,7 @@ HISTOGRAM(RANGE_CHECKSUM_ERRORS, 1, 3000, 10, EXPONENTIAL, "Number of histograms
 HISTOGRAM(BUCKET_ORDER_ERRORS, 1, 3000, 10, EXPONENTIAL, "Number of histograms with bucket order errors")
 HISTOGRAM(TOTAL_COUNT_HIGH_ERRORS, 1, 3000, 10, EXPONENTIAL, "Number of histograms with total count high errors")
 HISTOGRAM(TOTAL_COUNT_LOW_ERRORS, 1, 3000, 10, EXPONENTIAL, "Number of histograms with total count low errors")
+HISTOGRAM_FLAG(TELEMETRY_TEST_FLAG, "a testing histogram; not meant to be touched")
 
 #undef HISTOGRAM_BOOLEAN
 #undef HISTOGRAM_FLAG
diff --git a/toolkit/components/telemetry/tests/unit/test_TelemetryPing.js b/toolkit/components/telemetry/tests/unit/test_TelemetryPing.js
index 4b8c6aeef5c8..b743664aa58d 100644
--- a/toolkit/components/telemetry/tests/unit/test_TelemetryPing.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetryPing.js
@@ -121,6 +121,7 @@ function checkHistograms(request, response) {
 
   const TELEMETRY_PING = "TELEMETRY_PING";
   const TELEMETRY_SUCCESS = "TELEMETRY_SUCCESS";
+  const TELEMETRY_TEST_FLAG = "TELEMETRY_TEST_FLAG";
   do_check_true(TELEMETRY_PING in payload.histograms);
   let rh = Telemetry.registeredHistograms;
   for (let name in rh) {
@@ -131,6 +132,17 @@ function checkHistograms(request, response) {
   do_check_false(IGNORE_HISTOGRAM in payload.histograms);
   do_check_false(IGNORE_CLONED_HISTOGRAM in payload.histograms);
 
+  // Flag histograms should automagically spring to life.
+  const expected_flag = {
+    range: [1, 2],
+    bucket_count: 3,
+    histogram_type: 3,
+    values: {0:1, 1:0},
+    sum: 1
+  };
+  let flag = payload.histograms[TELEMETRY_TEST_FLAG];
+  do_check_eq(uneval(flag), uneval(expected_flag));
+
   // There should be one successful report from the previous telemetry ping.
   const expected_tc = {
     range: [1, 2],
diff --git a/toolkit/components/telemetry/tests/unit/test_nsITelemetry.js b/toolkit/components/telemetry/tests/unit/test_nsITelemetry.js
index 844f0471d43e..918462198729 100644
--- a/toolkit/components/telemetry/tests/unit/test_nsITelemetry.js
+++ b/toolkit/components/telemetry/tests/unit/test_nsITelemetry.js
@@ -85,6 +85,32 @@ function test_boolean_histogram()
   do_check_eq(s.counts[0], 2);
 }
 
+function test_flag_histogram()
+{
+  var h = Telemetry.newHistogram("test::flag histogram", 130, 4, 5, Telemetry.HISTOGRAM_FLAG);
+  var r = h.snapshot().ranges;
+  // Flag histograms ignore numeric parameters.
+  do_check_eq(uneval(r), uneval([0, 1, 2]))
+  // Should already have a 0 counted.
+  var c = h.snapshot().counts;
+  var s = h.snapshot().sum;
+  do_check_eq(uneval(c), uneval([1, 0, 0]));
+  do_check_eq(s, 1);
+  // Should switch counts.
+  h.add(2);
+  var c2 = h.snapshot().counts;
+  var s2 = h.snapshot().sum;
+  do_check_eq(uneval(c2), uneval([0, 1, 0]));
+  do_check_eq(s, 1);
+  // Should only switch counts once.
+  h.add(3);
+  var c3 = h.snapshot().counts;
+  var s3 = h.snapshot().sum;
+  do_check_eq(uneval(c3), uneval([0, 1, 0]));
+  do_check_eq(s3, 1);
+  do_check_eq(h.snapshot().histogram_type, Telemetry.FLAG_HISTOGRAM);
+}
+
 function test_getHistogramById() {
   try {
     Telemetry.getHistogramById("nonexistent");
@@ -148,10 +174,19 @@ function test_addons() {
   expect_success(function ()
 		 register(extra_addon, name1, 0, 1, 2, Telemetry.HISTOGRAM_BOOLEAN));
 
+  // Check that we can register flag histograms.
+  var flag_addon = "testing-flag-addon";
+  var flag_histogram = "flag-histogram";
+  expect_success(function() 
+                 register(flag_addon, flag_histogram, 0, 1, 2, Telemetry.HISTOGRAM_FLAG))
+  expect_success(function()
+		 register(flag_addon, name2, 2, 4, 4, Telemetry.HISTOGRAM_LINEAR));
+
   // Check that we reflect registered addons and histograms.
   snapshots = Telemetry.addonHistogramSnapshots;
   do_check_true(addon_id in snapshots)
   do_check_true(extra_addon in snapshots);
+  do_check_true(flag_addon in snapshots);
 
   // Check that we have data for our created histograms.
   do_check_true(name1 in snapshots[addon_id]);
@@ -168,6 +203,10 @@ function test_addons() {
   // Even though we've registered it, it shouldn't show up until data is added to it.
   do_check_false(name1 in snapshots[extra_addon]);
 
+  // Flag histograms should show up automagically.
+  do_check_true(flag_histogram in snapshots[flag_addon]);
+  do_check_false(name2 in snapshots[flag_addon]);
+
   // Check that we can remove addon histograms.
   Telemetry.unregisterAddonHistograms(addon_id);
   snapshots = Telemetry.addonHistogramSnapshots;