--- a/toolkit/components/extensions/ext-browserSettings.js
+++ b/toolkit/components/extensions/ext-browserSettings.js
@@ -1,21 +1,27 @@
 /* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
 /* vim: set sts=2 sw=2 et tw=80: */
 "use strict";
 
+XPCOMUtils.defineLazyModuleGetter(this, "AppConstants",
+                                  "resource://gre/modules/AppConstants.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "Services",
                                   "resource://gre/modules/Services.jsm");
 
 XPCOMUtils.defineLazyServiceGetter(this, "aboutNewTabService",
                                    "@mozilla.org/browser/aboutnewtab-service;1",
                                    "nsIAboutNewTabService");
 
 Cu.import("resource://gre/modules/ExtensionPreferencesManager.jsm");
 
+var {
+  ExtensionError,
+} = ExtensionUtils;
+
 const HOMEPAGE_OVERRIDE_SETTING = "homepage_override";
 const HOMEPAGE_URL_PREF = "browser.startup.homepage";
 const URL_STORE_TYPE = "url_overrides";
 const NEW_TAB_OVERRIDE_SETTING = "newTabURL";
 
 const PERM_DENY_ACTION = Services.perms.DENY_ACTION;
 
 const getSettingsAPI = (extension, name, callback, storeType, readOnly = false) => {
@@ -77,16 +83,26 @@ ExtensionPreferencesManager.addSetting("
     "image.animation_mode",
   ],
 
   setCallback(value) {
     return {[this.prefNames[0]]: value};
   },
 });
 
+ExtensionPreferencesManager.addSetting("contextMenuShowEvent", {
+  prefNames: [
+    "ui.context_menus.after_mouseup",
+  ],
+
+  setCallback(value) {
+    return {[this.prefNames[0]]: value === "mouseup"};
+  },
+});
+
 ExtensionPreferencesManager.addSetting("webNotificationsDisabled", {
   prefNames: [
     "permissions.default.desktop-notification",
   ],
 
   setCallback(value) {
     return {[this.prefNames[0]]: value ? PERM_DENY_ACTION : undefined};
   },
@@ -119,16 +135,45 @@ this.browserSettings = class extends Ext
           () => {
             return Services.prefs.getCharPref("image.animation_mode");
           }),
         newTabPageOverride: getSettingsAPI(extension,
           NEW_TAB_OVERRIDE_SETTING,
           () => {
             return aboutNewTabService.newTabURL;
           }, URL_STORE_TYPE, true),
+        contextMenuShowEvent: Object.assign(
+          getSettingsAPI(
+            extension,
+            "contextMenuShowEvent",
+            () => {
+              if (AppConstants.platform === "win") {
+                return "mouseup";
+              }
+              let prefValue = Services.prefs.getBoolPref(
+                "ui.context_menus.after_mouseup", null);
+              return prefValue ? "mouseup" : "mousedown";
+            }
+          ),
+          {
+            set: details => {
+              if (!["mouseup", "mousedown"].includes(details.value)) {
+                throw new ExtensionError(
+                  `${details.value} is not a valid value for contextMenuShowEvent.`);
+              }
+              if (AppConstants.platform === "android" ||
+                  (AppConstants.platform === "win" &&
+                   details.value === "mousedown")) {
+                return false;
+              }
+              return ExtensionPreferencesManager.setSetting(
+                extension.id, "contextMenuShowEvent", details.value);
+            },
+          }
+        ),
         webNotificationsDisabled: getSettingsAPI(extension,
           "webNotificationsDisabled",
           () => {
             let prefValue =
               Services.prefs.getIntPref(
                 "permissions.default.desktop-notification", null);
             return prefValue === PERM_DENY_ACTION;
           }),

--- a/toolkit/components/extensions/schemas/browser_settings.json
+++ b/toolkit/components/extensions/schemas/browser_settings.json
@@ -22,16 +22,22 @@
     "description": "Use the <code>browser.browserSettings</code> API to control global settings of the browser.",
     "permissions": ["browserSettings"],
     "types": [
       {
         "id": "ImageAnimationBehavior",
         "type": "string",
         "enum": ["normal", "none", "once"],
         "description": "How images should be animated in the browser."
+      },
+      {
+        "id": "ContextMenuMouseEvent",
+        "type": "string",
+        "enum": ["mouseup", "mousedown"],
+        "description": "After which mouse event context menus should popup."
       }
     ],
     "properties": {
       "allowPopupsForUserEvents": {
         "$ref": "types.Setting",
         "description": "Allows or disallows pop-up windows from opening in response to user events."
       },
       "cacheEnabled": {
@@ -45,15 +51,19 @@
       "imageAnimationBehavior": {
         "$ref": "types.Setting",
         "description": "Controls the behaviour of image animation in the browser. This setting's value is of type ImageAnimationBehavior, defaulting to <code>normal</code>."
       },
       "newTabPageOverride": {
         "$ref": "types.Setting",
         "description": "Returns the value of the overridden new tab page. Read-only."
       },
+      "contextMenuShowEvent": {
+        "$ref": "types.Setting",
+        "description": "Controls after which mouse event context menus popup. This setting's value is of type ContextMenuMouseEvent, which has possible values of <code>mouseup</code> and <code>mousedown</code>."
+      },
       "webNotificationsDisabled": {
         "$ref": "types.Setting",
         "description": "Disables webAPI notifications."
       }
     }
   }
 ]

--- a/toolkit/components/extensions/test/xpcshell/test_ext_browserSettings.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_browserSettings.js
@@ -23,23 +23,30 @@ add_task(async function test_browser_set
 
   // Create an object to hold the values to which we will initialize the prefs.
   const PREFS = {
     "browser.cache.disk.enable": true,
     "browser.cache.memory.enable": true,
     "dom.popup_allowed_events": Preferences.get("dom.popup_allowed_events"),
     "image.animation_mode": "none",
     "permissions.default.desktop-notification": PERM_UNKNOWN_ACTION,
+    "ui.context_menus.after_mouseup": false,
   };
 
   async function background() {
     browser.test.onMessage.addListener(async (msg, apiName, value) => {
       let apiObj = browser.browserSettings[apiName];
-      await apiObj.set({value});
-      browser.test.sendMessage("settingData", await apiObj.get({}));
+      let result = await apiObj.set({value});
+      if (msg === "set") {
+        browser.test.assertTrue(result, "set returns true.");
+        browser.test.sendMessage("settingData", await apiObj.get({}));
+      } else {
+        browser.test.assertFalse(result, "set returns false for a no-op.");
+        browser.test.sendMessage("no-op set");
+      }
     });
   }
 
   // Set prefs to our initial values.
   for (let pref in PREFS) {
     Preferences.set(pref, PREFS[pref]);
   }
 
@@ -68,16 +75,24 @@ add_task(async function test_browser_set
           `The ${setting} setting has the expected value.`);
     equal(data.levelOfControl, "controlled_by_this_extension",
           `The ${setting} setting has the expected levelOfControl.`);
     for (let pref in expected) {
       equal(Preferences.get(pref), expected[pref], `${pref} set correctly for ${value}`);
     }
   }
 
+  async function testNoOpSetting(setting, value, expected) {
+    extension.sendMessage("setNoOp", setting, value);
+    await extension.awaitMessage("no-op set");
+    for (let pref in expected) {
+      equal(Preferences.get(pref), expected[pref], `${pref} set correctly for ${value}`);
+    }
+  }
+
   await testSetting(
     "cacheEnabled", false,
     {
       "browser.cache.disk.enable": false,
       "browser.cache.memory.enable": false,
     });
   await testSetting(
     "cacheEnabled", true,
@@ -105,12 +120,53 @@ add_task(async function test_browser_set
   await testSetting(
     "webNotificationsDisabled", false,
     {
       // This pref is not defaulted on Android.
       "permissions.default.desktop-notification":
         AppConstants.MOZ_BUILD_APP !== "browser" ? undefined : PERM_UNKNOWN_ACTION,
     });
 
+  // This setting is a no-op on Android.
+  if (AppConstants.platform === "android") {
+    await testNoOpSetting("contextMenuShowEvent", "mouseup",
+      {"ui.context_menus.after_mouseup": false});
+  } else {
+    await testSetting(
+      "contextMenuShowEvent", "mouseup",
+      {"ui.context_menus.after_mouseup": true});
+  }
+
+  // "mousedown" is also a no-op on Windows.
+  if (["android", "win"].includes(AppConstants.platform)) {
+    await testNoOpSetting("contextMenuShowEvent", "mousedown",
+      {"ui.context_menus.after_mouseup": AppConstants.platform === "win"});
+  } else {
+    await testSetting(
+      "contextMenuShowEvent", "mousedown",
+      {"ui.context_menus.after_mouseup": false});
+  }
+
   await extension.unload();
-
   await promiseShutdownManager();
 });
+
+add_task(async function test_bad_value() {
+  async function background() {
+    await browser.test.assertRejects(
+      browser.browserSettings.contextMenuShowEvent.set({value: "bad"}),
+      /bad is not a valid value for contextMenuShowEvent/,
+      "contextMenuShowEvent.set rejects with an invalid value.");
+
+    browser.test.sendMessage("done");
+  }
+
+  let extension = ExtensionTestUtils.loadExtension({
+    background,
+    manifest: {
+      permissions: ["browserSettings"],
+    },
+  });
+
+  await extension.startup();
+  await extension.awaitMessage("done");
+  await extension.unload();
+});