diff --git a/manifest.json b/manifest.json
index e8fe3ec..9e82282 100644
--- a/manifest.json
+++ b/manifest.json
@@ -26,13 +26,6 @@
         "32": "assets/icon-addon.svg",
         "64": "assets/icon-addon.svg"
     },
-    "content_scripts": [
-        {
-            "matches": ["<all_urls>"],
-            "js": ["lib/tests/test-content-script.js"],
-            "run_at": "document_end"
-        }
-    ],
     "permissions": [
         "proxy",
         "storage",
diff --git a/tests/test-lib.js b/tests/test-lib.js
index 893a92c..6ab61d4 100644
--- a/tests/test-lib.js
+++ b/tests/test-lib.js
@@ -1,14 +1,36 @@
 // Test library for Marionette-based extension verification
 // This JavaScript code runs in the browser context via Marionette
 
+/**
+ * Inject test content script into the page
+ * @returns {Promise<boolean>} - True if injection successful
+ */
+async function injectTestContentScript() {
+    // Read the content script file
+    const response = await fetch(browser.runtime.getURL('lib/tests/test-content-script.js'));
+    const scriptCode = await response.text();
+
+    // Inject it into the page
+    const script = document.createElement('script');
+    script.textContent = scriptCode;
+    document.documentElement.appendChild(script);
+    script.remove();
+
+    // Wait a bit for script to initialize
+    await new Promise(resolve => setTimeout(resolve, 100));
+
+    return document.body.getAttribute('data-rentgen-injected') === 'true';
+}
+
 /**
  * Test that background script performs computation correctly
  * @param {number} testValue - Input value for computation
  * @returns {Promise<number|null>} - Computed result or null on failure
  */
-function testBackgroundComputation(testValue) {
-    // Check if content script loaded
-    if (!document.body.getAttribute('data-rentgen-injected')) {
+async function testBackgroundComputation(testValue) {
+    // Inject content script first
+    const injected = await injectTestContentScript();
+    if (!injected) {
         return -1;  // Content script not loaded
     }