--- a/devtools/shared/moz.build
+++ b/devtools/shared/moz.build
@@ -58,13 +58,14 @@ DevToolsModules(
     'event-emitter.js',
     'flags.js',
     'indentation.js',
     'l10n.js',
     'loader-plugin-raw.jsm',
     'Loader.jsm',
     'Parser.jsm',
     'path.js',
+    'plural-form.js',
     'protocol.js',
     'system.js',
     'task.js',
     'ThreadSafeDevToolsUtils.js',
 )

new file mode 100644
--- /dev/null
+++ b/devtools/shared/plural-form.js
@@ -0,0 +1,196 @@
+/* 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/. */
+
+/*
+ * The code below is mostly is a slight modification of intl/locale/PluralForm.jsm that
+ * removes dependencies on chrome privileged APIs. To make maintenance easier, this file
+ * is kept as close as possible to the original in terms of implementation.
+ * The modified methods here are
+ * - makeGetter (remove code adding the caller name to the log)
+ * - get ruleNum() (rely on LocalizationHelper instead of String.services)
+ * - log() (rely on console.log)
+ *
+ * Disable eslint warnings to preserve original code style.
+ */
+
+/* eslint-disable */
+
+/**
+ * This module provides the PluralForm object which contains a method to figure
+ * out which plural form of a word to use for a given number based on the
+ * current localization. There is also a makeGetter method that creates a get
+ * function for the desired plural rule. This is useful for extensions that
+ * specify their own plural rule instead of relying on the browser default.
+ * (I.e., the extension hasn't been localized to the browser's locale.)
+ *
+ * See: http://developer.mozilla.org/en/docs/Localization_and_Plurals
+ *
+ * List of methods:
+ *
+ * string pluralForm
+ * get(int aNum, string aWords)
+ *
+ * int numForms
+ * numForms()
+ *
+ * [string pluralForm get(int aNum, string aWords), int numForms numForms()]
+ * makeGetter(int aRuleNum)
+ * Note: Basically, makeGetter returns 2 functions that do "get" and "numForm"
+ */
+
+const {LocalizationHelper} = require("devtools/shared/l10n");
+const L10N = new LocalizationHelper("global/locale/intl.properties");
+
+// These are the available plural functions that give the appropriate index
+// based on the plural rule number specified. The first element is the number
+// of plural forms and the second is the function to figure out the index.
+var gFunctions = [
+  // 0: Chinese
+  [1, (n) => 0],
+  // 1: English
+  [2, (n) => n!=1?1:0],
+  // 2: French
+  [2, (n) => n>1?1:0],
+  // 3: Latvian
+  [3, (n) => n%10==1&&n%100!=11?1:n!=0?2:0],
+  // 4: Scottish Gaelic
+  [4, (n) => n==1||n==11?0:n==2||n==12?1:n>0&&n<20?2:3],
+  // 5: Romanian
+  [3, (n) => n==1?0:n==0||n%100>0&&n%100<20?1:2],
+  // 6: Lithuanian
+  [3, (n) => n%10==1&&n%100!=11?0:n%10>=2&&(n%100<10||n%100>=20)?2:1],
+  // 7: Russian
+  [3, (n) => n%10==1&&n%100!=11?0:n%10>=2&&n%10<=4&&(n%100<10||n%100>=20)?1:2],
+  // 8: Slovak
+  [3, (n) => n==1?0:n>=2&&n<=4?1:2],
+  // 9: Polish
+  [3, (n) => n==1?0:n%10>=2&&n%10<=4&&(n%100<10||n%100>=20)?1:2],
+  // 10: Slovenian
+  [4, (n) => n%100==1?0:n%100==2?1:n%100==3||n%100==4?2:3],
+  // 11: Irish Gaeilge
+  [5, (n) => n==1?0:n==2?1:n>=3&&n<=6?2:n>=7&&n<=10?3:4],
+  // 12: Arabic
+  [6, (n) => n==0?5:n==1?0:n==2?1:n%100>=3&&n%100<=10?2:n%100>=11&&n%100<=99?3:4],
+  // 13: Maltese
+  [4, (n) => n==1?0:n==0||n%100>0&&n%100<=10?1:n%100>10&&n%100<20?2:3],
+  // 14: Macedonian
+  [3, (n) => n%10==1?0:n%10==2?1:2],
+  // 15: Icelandic
+  [2, (n) => n%10==1&&n%100!=11?0:1],
+  // 16: Breton
+  [5, (n) => n%10==1&&n%100!=11&&n%100!=71&&n%100!=91?0:n%10==2&&n%100!=12&&n%100!=72&&n%100!=92?1:(n%10==3||n%10==4||n%10==9)&&n%100!=13&&n%100!=14&&n%100!=19&&n%100!=73&&n%100!=74&&n%100!=79&&n%100!=93&&n%100!=94&&n%100!=99?2:n%1000000==0&&n!=0?3:4],
+];
+
+this.PluralForm = {
+  /**
+   * Get the correct plural form of a word based on the number
+   *
+   * @param aNum
+   *        The number to decide which plural form to use
+   * @param aWords
+   *        A semi-colon (;) separated string of words to pick the plural form
+   * @return The appropriate plural form of the word
+   */
+  get get()
+  {
+    // This method will lazily load to avoid perf when it is first needed and
+    // creates getPluralForm function. The function it creates is based on the
+    // value of pluralRule specified in the intl stringbundle.
+    // See: http://developer.mozilla.org/en/docs/Localization_and_Plurals
+
+    // Delete the getters to be overwritten
+    delete PluralForm.numForms;
+    delete PluralForm.get;
+
+    // Make the plural form get function and set it as the default get
+    [PluralForm.get, PluralForm.numForms] = PluralForm.makeGetter(PluralForm.ruleNum);
+    return PluralForm.get;
+  },
+
+  /**
+   * Create a pair of plural form functions for the given plural rule number.
+   *
+   * @param aRuleNum
+   *        The plural rule number to create functions
+   * @return A pair: [function that gets the right plural form,
+   *                  function that returns the number of plural forms]
+   */
+  makeGetter: function(aRuleNum)
+  {
+    // Default to "all plural" if the value is out of bounds or invalid
+    if (aRuleNum < 0 || aRuleNum >= gFunctions.length || isNaN(aRuleNum)) {
+      log(["Invalid rule number: ", aRuleNum, " -- defaulting to 0"]);
+      aRuleNum = 0;
+    }
+
+    // Get the desired pluralRule function
+    let [numForms, pluralFunc] = gFunctions[aRuleNum];
+
+    // Return functions that give 1) the number of forms and 2) gets the right
+    // plural form
+    return [function(aNum, aWords) {
+      // Figure out which index to use for the semi-colon separated words
+      let index = pluralFunc(aNum ? Number(aNum) : 0);
+      let words = aWords ? aWords.split(/;/) : [""];
+
+      // Explicitly check bounds to avoid strict warnings
+      let ret = index < words.length ? words[index] : undefined;
+
+      // Check for array out of bounds or empty strings
+      if ((ret == undefined) || (ret == "")) {
+        // Display a message in the error console
+        log(["Index #", index, " of '", aWords, "' for value ", aNum,
+            " is invalid -- plural rule #", aRuleNum, ";"]);
+
+        // Default to the first entry (which might be empty, but not undefined)
+        ret = words[0];
+      }
+
+      return ret;
+    }, () => numForms];
+  },
+
+  /**
+   * Get the number of forms for the current plural rule
+   *
+   * @return The number of forms
+   */
+  get numForms()
+  {
+    // We lazily load numForms, so trigger the init logic with get()
+    PluralForm.get();
+    return PluralForm.numForms;
+  },
+
+  /**
+   * Get the plural rule number from the intl stringbundle
+   *
+   * @return The plural rule number
+   */
+  get ruleNum()
+  {
+    try {
+      return parseInt(L10N.getStr("pluralRule"), 10);
+    } catch (e) {
+      // Fallback to English if the pluralRule property is not available.
+      return 1;
+    }
+  }
+};
+
+/**
+ * Private helper function to log errors to the error console and command line
+ *
+ * @param aMsg
+ *        Error message to log or an array of strings to concat
+ */
+function log(aMsg)
+{
+  let msg = "plural-form.js: " + (aMsg.join ? aMsg.join("") : aMsg);
+  console.log(msg + "\n");
+}
+
+exports.PluralForm = this.PluralForm;
+
+/* eslint-ensable */

new file mode 100644
--- /dev/null
+++ b/devtools/shared/tests/unit/test_pluralForm-english.js
@@ -0,0 +1,29 @@
+/* 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/. */
+
+"use strict";
+
+/**
+ * This unit test makes sure the plural form for Irish Gaeilge is working by
+ * using the makeGetter method instead of using the default language (by
+ * development), English.
+ */
+
+const {PluralForm} = require("devtools/shared/plural-form");
+
+function run_test() {
+  // English has 2 plural forms
+  do_check_eq(2, PluralForm.numForms());
+
+  // Make sure for good inputs, things work as expected
+  for (let num = 0; num <= 200; num++) {
+    do_check_eq(num == 1 ? "word" : "words", PluralForm.get(num, "word;words"));
+  }
+
+  // Not having enough plural forms defaults to the first form
+  do_check_eq("word", PluralForm.get(2, "word"));
+
+  // Empty forms defaults to the first form
+  do_check_eq("word", PluralForm.get(2, "word;"));
+}

new file mode 100644
--- /dev/null
+++ b/devtools/shared/tests/unit/test_pluralForm-makeGetter.js
@@ -0,0 +1,38 @@
+/* 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/. */
+
+"use strict";
+
+/**
+ * This unit test makes sure the plural form for Irish Gaeilge is working by
+ * using the makeGetter method instead of using the default language (by
+ * development), English.
+ */
+
+const {PluralForm} = require("devtools/shared/plural-form");
+
+function run_test() {
+  // Irish is plural rule #11
+  let [get, numForms] = PluralForm.makeGetter(11);
+
+  // Irish has 5 plural forms
+  do_check_eq(5, numForms());
+
+  // I don't really know Irish, so I'll stick in some dummy text
+  let words = "is 1;is 2;is 3-6;is 7-10;everything else";
+
+  let test = function (text, low, high) {
+    for (let num = low; num <= high; num++) {
+      do_check_eq(text, get(num, words));
+    }
+  };
+
+  // Make sure for good inputs, things work as expected
+  test("everything else", 0, 0);
+  test("is 1", 1, 1);
+  test("is 2", 2, 2);
+  test("is 3-6", 3, 6);
+  test("is 7-10", 7, 10);
+  test("everything else", 11, 200);
+}

--- a/devtools/shared/tests/unit/xpcshell.ini
+++ b/devtools/shared/tests/unit/xpcshell.ini
@@ -18,15 +18,17 @@ support-files =
 [test_indentation.js]
 [test_independent_loaders.js]
 [test_invisible_loader.js]
 [test_isSet.js]
 [test_safeErrorString.js]
 [test_defineLazyPrototypeGetter.js]
 [test_async-utils.js]
 [test_console_filtering.js]
+[test_pluralForm-english.js]
+[test_pluralForm-makeGetter.js]
 [test_prettifyCSS.js]
 [test_require_lazy.js]
 [test_require_raw.js]
 [test_require.js]
 [test_stack.js]
 [test_defer.js]
 [test_executeSoon.js]