diff --git a/toolkit/components/extensions/ext-contentScripts.js b/toolkit/components/extensions/ext-contentScripts.js
index febaf44c94ce..717a60f51faa 100644
--- a/toolkit/components/extensions/ext-contentScripts.js
+++ b/toolkit/components/extensions/ext-contentScripts.js
@@ -68,7 +68,7 @@ class ContentScriptParent {
       exclude_globs: details.excludeGlobs,
       all_frames: details.allFrames,
       match_about_blank: details.matchAboutBlank,
-      run_at: details.runAt,
+      run_at: details.runAt || "document_idle",
       js: [],
       css: [],
     };
diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_contentScripts_register.js b/toolkit/components/extensions/test/xpcshell/test_ext_contentScripts_register.js
index 42981202a2ac..118ba9416208 100644
--- a/toolkit/components/extensions/test/xpcshell/test_ext_contentScripts_register.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_contentScripts_register.js
@@ -308,7 +308,6 @@ add_task(async function test_contentscripts_register_js() {
         js: [{code: `(${textScriptCodeIdle})()`}],
         runAt: "document_idle",
       },
-
       // Extension URLs.
       {
         matches: ["http://localhost/*/file_sample.html"],
@@ -328,7 +327,7 @@ add_task(async function test_contentscripts_register_js() {
       {
         matches: ["http://localhost/*/file_sample.html"],
         js: [{file: "content_script.js"}],
-        runAt: "document_idle",
+        // "runAt" is not specified here to ensure that it defaults to document_idle when missing.
       },
     ];