You can follow the following steps to install selenium IDE:-
- Start Firefox browser, download selenium IDE from the www.seleniumhq.org website.
 
- Click "Allow" to proceed with the installation.
 
- Install multiple items to support Java, Ruby, Python etc... formats. 
 
- Restart Firefox browser.
 
- After Firefox reboots then we can use Selenium IDE.
 
That's all, you have installed selenium IDE.