Unfortunately, Selenium IDE will not work anymore with latest versions of Firefox. From v55 of Firefox, IDE will not work. If you still want to use it, downgrade your Selenium IDE to older versions and you'll be able to do it. But this is not recommended because eventually, support for Selenium IDE will be stopped.
A long time solution would rather be to explore other tools like:
- Robot Framework
- Protractor.JS
- katalon studio