Open the Firefox Add-ons Manager by selecting the Add-ons menu item from the Tools menu. The Add-ons Manager will open in a new tab. In the Add-ons Manager, click on the Extensions item from the list usually on the left hand side. This should show you all the extensions installed.
Now search in the extensions list for the Selenium IDE. It should be shown as Selenium IDE followed by the version of Selenium IDE. If it is disabled, the name will contain (disabled) and there will be a Enable button.
If Selenium IDE is disabled, you can enable Selenium IDE again, click on the Enable button. Firefox will show you a link to Restart now on the addon. Click on this Restart now link to restart Firefox. When Firefox has restarted, check if Selenium IDE has been enabled.