Firefox - webRequest.onBeforeSendHeaders

Question

I'm getting this in Firefox while in Chrome the same works

Error: Type error for parameter filter (Error processing urls.0: Expected string instead of undefined) for webRequest.onBeforeSendHeaders.

    import browser from "webextension-polyfill";
    
    browser.webRequest.onBeforeSendHeaders.addListener(
      listener,
      {
        urls: [url],
      },
      ["requestHeaders", "blocking", "extraHeaders"]
    );

I verified the url is a string, any idea where this is from?