Greasemonkey is the popular and most used add-on for Mozilla Firefox web browsers, with this add-on Mozilla Firefox users can install and run different scripts that helps to modify some functionality of websites you visit.
Opera has a built-in feature that supports Greasemonkey scripts, that feature is known as custom JavaScript.
How to use scripts of Greasemonkey in Opera
- Download any script of Greasemonkey from here
- Rename the downloaded script so that it ends with ".user.js"
- In Opera web browser, go to Tools and then Preferences
- From the opened Preferences window, go to Advanced tab and then select Content
- Now click on the JavaScript options button

- In User JavaScript files field, click on Choose to select the location of folder where script is saved
- Changes will take effect after restarting Opera web browser
