You can install Selenium IDE plug-in on Firefox by following the steps below:
(1) Open the site "http://seleniumhq.org" on your Firefox Browser.
(2) Click on the "Download" button on the top navigation menu.
(3) Find the Selenium IDE paragraph on this page, and then click on the link in blue: Download version 1.8.1 released on 01/June/2012.
(5) click on the button install to continue the installation.
(6) Restart Firefox to complete the Selenium IDE Plug-in installation.
Now, you can use Selenium IDE for test case and suite automation.