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diff --git a/toolkit/components/url-classifier/UrlClassifierLib.sys.mjs b/toolkit/components/url-classifier/UrlClassifierLib.sys.mjs
new file mode 100644
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+++ b/toolkit/components/url-classifier/UrlClassifierLib.sys.mjs
@@ -0,0 +1,227 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// We wastefully reload the same JS files across components.  This puts all
+// the common JS files used by safebrowsing and url-classifier into a
+// single component.
+
+const PREF_DISABLE_TEST_BACKOFF =
+  "browser.safebrowsing.provider.test.disableBackoff";
+
+/**
+ * Partially applies a function to a particular "this object" and zero or
+ * more arguments. The result is a new function with some arguments of the first
+ * function pre-filled and the value of |this| "pre-specified".
+ *
+ * Remaining arguments specified at call-time are appended to the pre-
+ * specified ones.
+ *
+ * Usage:
+ * var barMethBound = BindToObject(myFunction, myObj, "arg1", "arg2");
+ * barMethBound("arg3", "arg4");
+ *
+ * @param fn {string} Reference to the function to be bound
+ *
+ * @param self {object} Specifies the object which |this| should point to
+ * when the function is run. If the value is null or undefined, it will default
+ * to the global object.
+ *
+ * @returns {function} A partially-applied form of the speficied function.
+ */
+export function BindToObject(fn, self, opt_args) {
+  var boundargs = fn.boundArgs_ || [];
+  boundargs = boundargs.concat(
+    Array.prototype.slice.call(arguments, 2, arguments.length)
+  );
+
+  if (fn.boundSelf_) {
+    self = fn.boundSelf_;
+  }
+  if (fn.boundFn_) {
+    fn = fn.boundFn_;
+  }
+
+  var newfn = function () {
+    // Combine the static args and the new args into one big array
+    var args = boundargs.concat(Array.prototype.slice.call(arguments));
+    return fn.apply(self, args);
+  };
+
+  newfn.boundArgs_ = boundargs;
+  newfn.boundSelf_ = self;
+  newfn.boundFn_ = fn;
+
+  return newfn;
+}
+
+// This implements logic for stopping requests if the server starts to return
+// too many errors.  If we get MAX_ERRORS errors in ERROR_PERIOD minutes, we
+// back off for TIMEOUT_INCREMENT minutes.  If we get another error
+// immediately after we restart, we double the timeout and add
+// TIMEOUT_INCREMENT minutes, etc.
+//
+// This is similar to the logic used by the search suggestion service.
+
+// HTTP responses that count as an error.  We also include any 5xx response
+// as an error.
+const HTTP_FOUND = 302;
+const HTTP_SEE_OTHER = 303;
+const HTTP_TEMPORARY_REDIRECT = 307;
+
+/**
+ * @param maxErrors Number of times to request before backing off.
+ * @param retryIncrement Time (ms) for each retry before backing off.
+ * @param maxRequests Number the number of requests needed to trigger backoff
+ * @param requestPeriod Number time (ms) in which maxRequests have to occur to
+ *     trigger the backoff behavior (0 to disable maxRequests)
+ * @param timeoutIncrement Number time (ms) the starting timeout period
+ *     we double this time for consecutive errors
+ * @param maxTimeout Number time (ms) maximum timeout period
+ * @param tolerance Checking next request tolerance.
+ */
+function RequestBackoff(
+  maxErrors,
+  retryIncrement,
+  maxRequests,
+  requestPeriod,
+  timeoutIncrement,
+  maxTimeout,
+  tolerance,
+  provider = null
+) {
+  this.MAX_ERRORS_ = maxErrors;
+  this.RETRY_INCREMENT_ = retryIncrement;
+  this.MAX_REQUESTS_ = maxRequests;
+  this.REQUEST_PERIOD_ = requestPeriod;
+  this.TIMEOUT_INCREMENT_ = timeoutIncrement;
+  this.MAX_TIMEOUT_ = maxTimeout;
+  this.TOLERANCE_ = tolerance;
+
+  // Queue of ints keeping the time of all requests
+  this.requestTimes_ = [];
+
+  this.numErrors_ = 0;
+  this.errorTimeout_ = 0;
+  this.nextRequestTime_ = 0;
+
+  // For test provider, we will disable backoff if preference is set to false.
+  if (provider === "test") {
+    this.canMakeRequestDefault = this.canMakeRequest;
+    this.canMakeRequest = function () {
+      if (Services.prefs.getBoolPref(PREF_DISABLE_TEST_BACKOFF, true)) {
+        return true;
+      }
+      return this.canMakeRequestDefault();
+    };
+  }
+}
+
+/**
+ * Reset the object for reuse. This deliberately doesn't clear requestTimes_.
+ */
+RequestBackoff.prototype.reset = function () {
+  this.numErrors_ = 0;
+  this.errorTimeout_ = 0;
+  this.nextRequestTime_ = 0;
+};
+
+/**
+ * Check to see if we can make a request.
+ */
+RequestBackoff.prototype.canMakeRequest = function () {
+  var now = Date.now();
+  // Note that nsITimer delay is approximate: the timer can be fired before the
+  // requested time has elapsed. So, give it a tolerance
+  if (now + this.TOLERANCE_ < this.nextRequestTime_) {
+    return false;
+  }
+
+  return (
+    this.requestTimes_.length < this.MAX_REQUESTS_ ||
+    now - this.requestTimes_[0] > this.REQUEST_PERIOD_
+  );
+};
+
+RequestBackoff.prototype.noteRequest = function () {
+  var now = Date.now();
+  this.requestTimes_.push(now);
+
+  // We only care about keeping track of MAX_REQUESTS
+  if (this.requestTimes_.length > this.MAX_REQUESTS_) {
+    this.requestTimes_.shift();
+  }
+};
+
+RequestBackoff.prototype.nextRequestDelay = function () {
+  return Math.max(0, this.nextRequestTime_ - Date.now());
+};
+
+/**
+ * Notify this object of the last server response.  If it's an error,
+ */
+RequestBackoff.prototype.noteServerResponse = function (status) {
+  if (this.isErrorStatus(status)) {
+    this.numErrors_++;
+
+    if (this.numErrors_ < this.MAX_ERRORS_) {
+      this.errorTimeout_ = this.RETRY_INCREMENT_;
+    } else if (this.numErrors_ == this.MAX_ERRORS_) {
+      this.errorTimeout_ = this.TIMEOUT_INCREMENT_;
+    } else {
+      this.errorTimeout_ *= 2;
+    }
+
+    this.errorTimeout_ = Math.min(this.errorTimeout_, this.MAX_TIMEOUT_);
+    this.nextRequestTime_ = Date.now() + this.errorTimeout_;
+  } else {
+    // Reset error timeout, allow requests to go through.
+    this.reset();
+  }
+};
+
+/**
+ * We consider 302, 303, 307, 4xx, and 5xx http responses to be errors.
+ * @param status Number http status
+ * @return Boolean true if we consider this http status an error
+ */
+RequestBackoff.prototype.isErrorStatus = function (status) {
+  return (
+    (400 <= status && status <= 599) ||
+    HTTP_FOUND == status ||
+    HTTP_SEE_OTHER == status ||
+    HTTP_TEMPORARY_REDIRECT == status
+  );
+};
+
+// Wrap a general-purpose |RequestBackoff| to a v4-specific one
+// since both listmanager and hashcompleter would use it.
+// Note that |maxRequests| and |requestPeriod| is still configurable
+// to throttle pending requests.
+/* exported RequestBackoffV4 */
+function RequestBackoffV4(maxRequests, requestPeriod, provider = null) {
+  let rand = Math.random();
+  let retryInterval = Math.floor(15 * 60 * 1000 * (rand + 1)); // 15 ~ 30 min.
+  let backoffInterval = Math.floor(30 * 60 * 1000 * (rand + 1)); // 30 ~ 60 min.
+
+  return new RequestBackoff(
+    2 /* max errors */,
+    retryInterval /* retry interval, 15~30 min */,
+    maxRequests /* num requests */,
+    requestPeriod /* request time, 60 min */,
+    backoffInterval /* backoff interval, 60 min */,
+    24 * 60 * 60 * 1000 /* max backoff, 24hr */,
+    1000 /* tolerance of 1 sec */,
+    provider /* provider name */
+  );
+}
+
+export function UrlClassifierLib() {
+  this.wrappedJSObject = {
+    RequestBackoff,
+    RequestBackoffV4,
+    BindToObject,
+  };
+}
+
+UrlClassifierLib.prototype.QueryInterface = ChromeUtils.generateQI([]);