userScripts.onBeforeScript

browser.userScripts.onBeforeScript.addListener(listener)
browser.userScripts.onBeforeScript.removeListener(listener)
browser.userScripts.onBeforeScript.hasListener(listener)

browser.userScripts.onBeforeScript.addListener((script) => {

  script // This is an API object that represents the user script
         // that is going to be executed.

  script.metadata // Access the user script metadata (returns the
                  // value of the scriptMetadata property from
                  // the call to userScripts.register.

  // Export some global properties into the user script sandbox
  // (this method has to be called synchronously from the
  // listener, otherwise the user script may have executed).
  script.defineGlobals({
    aGlobalPropertyAccessibleFromUserScriptCode: "prop value",

    myCustomAPIMethod(param1, param2) {
      // Custom methods exported from the API script can use
      // the WebExtensions APIs available to content scripts.
      browser.runtime.sendMessage(/* … */);
      // …

      return 123; // primitive values can be returned directly
      // …

      // Non primitive values have to be exported explicitly
      // using the export method provided by the script API
      // object
      return script.export({
        objKey1: {
          nestedProp: "nestedValue",
        },
        // Explicitly exported objects can also provide methods.
        objMethod() { /* … */ }
      })
    },

    async myAsyncMethod(param1, param2, param3) {
    // exported methods can also be declared as async
    },
  });
});