--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -3351,16 +3351,23 @@
   "URLCLASSIFIER_COMPLETE_REMOTE_STATUS": {
     "alert_emails": ["gcp@mozilla.com", "francois@mozilla.com"],
     "expires_in_version": "never",
     "kind": "enumerated",
     "n_values": 16,
     "bug_numbers": [1150921],
     "description": "Server HTTP status code from remote SafeBrowsing gethash lookups. (0=1xx, 1=200, 2=2xx, 3=204, 4=3xx, 5=400, 6=4xx, 7=403, 8=404, 9=408, 10=413, 11=5xx, 12=502|504|511, 13=503, 14=505, 15=Other)"
   },
+  "URLCLASSIFIER_COMPLETE_TIMEOUT": {
+    "alert_emails": ["gcp@mozilla.com", "francois@mozilla.com"],
+    "expires_in_version": "52",
+    "kind": "boolean",
+    "bug_numbers": [1172688],
+    "description": "This metric is recorded every time a gethash lookup is performed, `true` is recorded if the lookup times out."
+  },
   "CSP_DOCUMENTS_COUNT": {
     "alert_emails": ["seceng@mozilla.com"],
     "bug_numbers": [1252829],
     "expires_in_version": "55",
     "kind": "count",
     "description": "Number of unique pages that contain a CSP"
   },
   "CSP_UNSAFE_INLINE_DOCUMENTS_COUNT": {

--- a/toolkit/components/url-classifier/nsUrlClassifierHashCompleter.js
+++ b/toolkit/components/url-classifier/nsUrlClassifierHashCompleter.js
@@ -333,16 +333,17 @@ HashCompleterRequest.prototype = {
   },
 
   notify: function HCR_notify() {
     // If we haven't gotten onStopRequest, just cancel. This will call us
     // with onStopRequest since we implement nsIStreamListener on the
     // channel.
     if (this._channel && this._channel.isPending()) {
       dump("hashcompleter: cancelling request to " + this.gethashUrl + "\n");
+      Services.telemetry.getHistogramById("URLCLASSIFIER_COMPLETE_TIMEOUT").add(1);
       this._channel.cancel(Cr.NS_BINDING_ABORTED);
     }
   },
 
   // Creates an nsIChannel for the request and fills the body.
   openChannel: function HCR_openChannel() {
     let loadFlags = Ci.nsIChannel.INHIBIT_CACHING |
                     Ci.nsIChannel.LOAD_BYPASS_CACHE;
@@ -543,16 +544,17 @@ HashCompleterRequest.prototype = {
       }
     }
     let success = Components.isSuccessCode(aStatusCode);
     log('Received a ' + httpStatus + ' status code from the gethash server (success=' + success + ').');
 
     let histogram =
       Services.telemetry.getHistogramById("URLCLASSIFIER_COMPLETE_REMOTE_STATUS");
     histogram.add(httpStatusToBucket(httpStatus));
+    Services.telemetry.getHistogramById("URLCLASSIFIER_COMPLETE_TIMEOUT").add(0);
 
     // Notify the RequestBackoff once a response is received.
     this._completer.finishRequest(this.gethashUrl, httpStatus);
 
     if (success) {
       try {
         this.handleResponse();
       }