diff --git a/background.js b/background.js
index 9553ad8..7e55ed0 100644
--- a/background.js
+++ b/background.js
@@ -81,13 +81,13 @@ chrome.runtime.onConnect.addListener((port) => {
 
 // browserByte returns either "F" for Firefox or "C" for chrome.
 // Other browsers return "?".
+// Firefox aliases `chrome.*` to its `browser.*` APIs, so
+// `chrome.runtime.getURL` works on both; the returned scheme is what differs.
+// See https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Chrome_incompatibilities
 function browserByte() {
-  if (typeof chrome !== "undefined") {
-    if (typeof browser !== "undefined") {
-      return "F"; // Firefox supports both `chrome` and `browser`
-    }
-    return "C";
-  }
+  const url = chrome.runtime.getURL("");
+  if (url.startsWith("moz-extension://")) return "F";
+  if (url.startsWith("chrome-extension://")) return "C";
   return "?";
 }