--- a/toolkit/components/extensions/ext-privacy.js
+++ b/toolkit/components/extensions/ext-privacy.js
@@ -211,16 +211,26 @@ ExtensionPreferencesManager.addSetting("
         prefs["privacy.trackingprotection.pbmode.enabled"] = false;
         break;
     }
 
     return prefs;
   },
 });
 
+ExtensionPreferencesManager.addSetting("websites.upgradeMixedDisplayContent", {
+  prefNames: [
+    "security.mixed_content.upgrade_display_content",
+  ],
+
+  setCallback(value) {
+    return {[this.prefNames[0]]: value};
+  },
+});
+
 this.privacy = class extends ExtensionAPI {
   getAPI(context) {
     let {extension} = context;
     return {
       privacy: {
         network: {
           networkPredictionEnabled: getPrivacyAPI(
             extension, "network.networkPredictionEnabled",
@@ -301,14 +311,19 @@ this.privacy = class extends ExtensionAP
             () => {
               if (Preferences.get("privacy.trackingprotection.enabled")) {
                 return "always";
               } else if (Preferences.get("privacy.trackingprotection.pbmode.enabled")) {
                 return "private_browsing";
               }
               return "never";
             }),
+          upgradeMixedDisplayContent: getPrivacyAPI(
+            extension, "websites.upgradeMixedDisplayContent",
+            () => {
+              return Preferences.get("security.mixed_content.upgrade_display_content");
+            }),
 
         },
       },
     };
   }
 };

--- a/toolkit/components/extensions/schemas/privacy.json
+++ b/toolkit/components/extensions/schemas/privacy.json
@@ -124,12 +124,16 @@
       },
       "trackingProtectionMode": {
         "$ref": "types.Setting",
         "description": "Allow users to specify the mode for tracking protection. This setting's value is of type TrackingProtectionModeOption, defaulting to <code>private_browsing_only</code>."
       },
       "cookieConfig": {
         "$ref": "types.Setting",
         "description": "Allow users to specify the default settings for allowing cookies, as well as whether all cookies should be created as non-persistent cookies. This setting's value is of type CookieConfig."
+      },
+      "upgradeMixedDisplayContent": {
+        "$ref": "types.Setting",
+        "description": "If enabled, the browser will upgrade any passive mixed content on secure connections."
       }
     }
   }
 ]

--- a/toolkit/components/extensions/test/xpcshell/test_ext_privacy.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_privacy.js
@@ -242,16 +242,19 @@ add_task(async function test_privacy_oth
     },
     "websites.firstPartyIsolate": {
       "privacy.firstparty.isolate": true,
     },
     "websites.cookieConfig": {
       "network.cookie.cookieBehavior": cookieSvc.BEHAVIOR_ACCEPT,
       "network.cookie.lifetimePolicy": cookieSvc.ACCEPT_NORMALLY,
     },
+    "websites.upgradeMixedDisplayContent": {
+      "security.mixed_content.upgrade_display_content": true,
+    },
   };
 
   async function background() {
     browser.test.onMessage.addListener(async (msg, ...args) => {
       let data = args[0];
       // The second argument is the end of the api name,
       // e.g., "network.webRTCIPHandlingPolicy".
       let apiObj = args[1].split(".").reduce((o, i) => o[i], browser.privacy);
@@ -474,16 +477,26 @@ add_task(async function test_privacy_oth
     "websites.cookieConfig",
     {nonPersistentCookies: false},
     {
       "network.cookie.cookieBehavior": cookieSvc.BEHAVIOR_ACCEPT,
       "network.cookie.lifetimePolicy": cookieSvc.ACCEPT_NORMALLY,
     },
     {behavior: "allow_all", nonPersistentCookies: false},
   );
+  await testSetting(
+    "websites.upgradeMixedDisplayContent", false,
+    {
+      "security.mixed_content.upgrade_display_content": false,
+    });
+  await testSetting(
+    "websites.upgradeMixedDisplayContent", true,
+    {
+      "security.mixed_content.upgrade_display_content": true,
+    });
 
   await extension.unload();
 
   await promiseShutdownManager();
 });
 
 add_task(async function test_exceptions() {
   async function background() {