User-extensions are very easy to use with the selenium IDE.
1. Create your user extension and save it as user-extensions.js. While this name isn’t technically necessary, it’s good practice to keep things consistent.
2. Open Firefox and open Selenium-IDE.
3. Click on Tools, Options
4. In Selenium Core Extensions click on Browse and find the user-extensions. js file. Click on OK.
5. Your user-extension will not yet be loaded, you must close and restart Selenium-IDE.
6. In your empty test, create a new command, your user-extension should now be an options in the Commands dropdown.