diff --git a/browser/components/extensions/ext-tabs.js b/browser/components/extensions/ext-tabs.js
index 7ef95e9e60ab..a0cce53d921b 100644
--- a/browser/components/extensions/ext-tabs.js
+++ b/browser/components/extensions/ext-tabs.js
@@ -809,11 +809,11 @@ extensions.registerSchemaAPI("tabs", (extension, context) => {
       },
 
       insertCSS: function(tabId, details) {
-        return self.tabs._execute(tabId, details, "css", "insertCSS");
+        return self.tabs._execute(tabId, details, "css", "insertCSS").then(() => {});
       },
 
       removeCSS: function(tabId, details) {
-        return self.tabs._execute(tabId, details, "css", "removeCSS");
+        return self.tabs._execute(tabId, details, "css", "removeCSS").then(() => {});
       },
 
       connect: function(tabId, connectInfo) {
diff --git a/browser/components/extensions/test/browser/browser_ext_tabs_executeScript.js b/browser/components/extensions/test/browser/browser_ext_tabs_executeScript.js
index 29c67486d098..183ec07e42e8 100644
--- a/browser/components/extensions/test/browser/browser_ext_tabs_executeScript.js
+++ b/browser/components/extensions/test/browser/browser_ext_tabs_executeScript.js
@@ -21,7 +21,8 @@ add_task(function* testExecuteScript() {
         browser.tabs.executeScript({
           code: "42",
         }).then(result => {
-          browser.test.assertEq(42, result, "Expected callback result");
+          browser.test.assertEq(1, result.length, "Expected one callback result");
+          browser.test.assertEq(42, result[0], "Expected callback result");
         }),
 
         browser.tabs.executeScript({
@@ -37,13 +38,15 @@ add_task(function* testExecuteScript() {
         browser.tabs.executeScript({
           file: "script.js",
         }).then(result => {
-          browser.test.assertEq(undefined, result, "Expected callback result");
+          browser.test.assertEq(1, result.length, "Expected one callback result");
+          browser.test.assertEq(undefined, result[0], "Expected callback result");
         }),
 
         browser.tabs.executeScript({
           file: "script2.js",
         }).then(result => {
-          browser.test.assertEq(27, result, "Expected callback result");
+          browser.test.assertEq(1, result.length, "Expected one callback result");
+          browser.test.assertEq(27, result[0], "Expected callback result");
         }),
 
         browser.tabs.executeScript({
@@ -62,9 +65,10 @@ add_task(function* testExecuteScript() {
           code: "location.href;",
           runAt: "document_end",
         }).then(result => {
-          browser.test.assertTrue(typeof(result) == "string", "Result is a string");
+          browser.test.assertEq(1, result.length, "Expected callback result");
+          browser.test.assertEq("string", typeof result[0], "Result is a string");
 
-          browser.test.assertTrue(/\/file_iframe_document\.html$/.test(result), "Result is correct");
+          browser.test.assertTrue(/\/file_iframe_document\.html$/.test(result[0]), "Result is correct");
         }),
 
         browser.tabs.executeScript({
@@ -118,7 +122,7 @@ add_task(function* testExecuteScript() {
         browser.tabs.executeScript({
           code: "Promise.resolve(42)",
         }).then(result => {
-          browser.test.assertEq(42, result, "Got expected promise resolution value as result");
+          browser.test.assertEq(42, result[0], "Got expected promise resolution value as result");
         }),
 
         browser.tabs.executeScript({
@@ -138,19 +142,21 @@ add_task(function* testExecuteScript() {
           code: "location.href;",
           frameId: frames[0].frameId,
         }).then(result => {
-          browser.test.assertTrue(/\/file_iframe_document\.html$/.test(result), `Result for frameId[0] is correct: ${result}`);
+          browser.test.assertEq(1, result.length, "Expected one result");
+          browser.test.assertTrue(/\/file_iframe_document\.html$/.test(result[0]), `Result for frameId[0] is correct: ${result[0]}`);
         }),
 
         browser.tabs.executeScript({
           code: "location.href;",
           frameId: frames[1].frameId,
         }).then(result => {
-          browser.test.assertEq("http://mochi.test:8888/", result, "Result for frameId[1] is correct");
+          browser.test.assertEq(1, result.length, "Expected one result");
+          browser.test.assertEq("http://mochi.test:8888/", result[0], "Result for frameId[1] is correct");
         }),
 
         browser.tabs.create({url: "http://example.com/"}).then(tab => {
           return browser.tabs.executeScript(tab.id, {code: "location.href"}).then(result => {
-            browser.test.assertEq("http://example.com/", result, "Script executed correctly in new tab");
+            browser.test.assertEq("http://example.com/", result[0], "Script executed correctly in new tab");
 
             return browser.tabs.remove(tab.id);
           });
diff --git a/browser/components/extensions/test/browser/browser_ext_tabs_insertCSS.js b/browser/components/extensions/test/browser/browser_ext_tabs_insertCSS.js
index 20123f45d1a6..6a2101c3c8ae 100644
--- a/browser/components/extensions/test/browser/browser_ext_tabs_insertCSS.js
+++ b/browser/components/extensions/test/browser/browser_ext_tabs_insertCSS.js
@@ -49,7 +49,7 @@ add_task(function* testExecuteScript() {
         return browser.tabs.executeScript({
           code: `(${checkCSS})()`,
         });
-      }).then(result => {
+      }).then(([result]) => {
         browser.test.assertEq(background, result[0], "Expected background color");
         browser.test.assertEq(foreground, result[1], "Expected foreground color");
         return next();
diff --git a/browser/components/extensions/test/browser/browser_ext_tabs_reload_bypass_cache.js b/browser/components/extensions/test/browser/browser_ext_tabs_reload_bypass_cache.js
index 231718f51f34..640710d36b51 100644
--- a/browser/components/extensions/test/browser/browser_ext_tabs_reload_bypass_cache.js
+++ b/browser/components/extensions/test/browser/browser_ext_tabs_reload_bypass_cache.js
@@ -33,14 +33,14 @@ add_task(function* () {
         return awaitLoad(tabId);
       }).then(() => {
         return browser.tabs.executeScript(tabId, {code: "document.body.textContent"});
-      }).then(textContent => {
+      }).then(([textContent]) => {
         browser.test.assertEq("", textContent, "`textContent` should be empty when bypassCache=false");
         return browser.tabs.reload(tabId, {bypassCache: true});
       }).then(() => {
         return awaitLoad(tabId);
       }).then(() => {
         return browser.tabs.executeScript(tabId, {code: "document.body.textContent"});
-      }).then(textContent => {
+      }).then(([textContent]) => {
         let [pragma, cacheControl] = textContent.split(":");
         browser.test.assertEq("no-cache", pragma, "`pragma` should be set to `no-cache` when bypassCache is true");
         browser.test.assertEq("no-cache", cacheControl, "`cacheControl` should be set to `no-cache` when bypassCache is true");
diff --git a/browser/components/extensions/test/browser/browser_ext_tabs_removeCSS.js b/browser/components/extensions/test/browser/browser_ext_tabs_removeCSS.js
index bcaafb2d1159..c4bb9a753319 100644
--- a/browser/components/extensions/test/browser/browser_ext_tabs_removeCSS.js
+++ b/browser/components/extensions/test/browser/browser_ext_tabs_removeCSS.js
@@ -66,7 +66,7 @@ add_task(function* testExecuteScript() {
         return browser.tabs.executeScript({
           code: `(${checkCSS})()`,
         });
-      }).then(result => {
+      }).then(([result]) => {
         browser.test.assertEq(background, result[0], "Expected background color");
         browser.test.assertEq(foreground, result[1], "Expected foreground color");
         return next();
diff --git a/toolkit/components/extensions/ExtensionContent.jsm b/toolkit/components/extensions/ExtensionContent.jsm
index 6fa31a45a341..0bdeaca25c31 100644
--- a/toolkit/components/extensions/ExtensionContent.jsm
+++ b/toolkit/components/extensions/ExtensionContent.jsm
@@ -614,13 +614,10 @@ DocumentManager = {
 
       return Promise.reject({message: `No window matching ${JSON.stringify(details)}`});
     }
-    if (options.all_frames) {
-      return Promise.all(promises);
-    }
-    if (promises.length > 1) {
+    if (!options.all_frames && promises.length > 1) {
       return Promise.reject({message: `Internal error: Script matched multiple windows`});
     }
-    return promises[0];
+    return Promise.all(promises);
   },
 
   enumerateWindows: function* (docShell) {