All add-ons can be found by clicking on the menu in the right corner - http://prntscr.com/9orlh6 then click "Add-ons" and just search the add-on you wish to use if you're on Windows. I don't believe you can use AdBlock, but Ghostery has an app in the Google Play store.

JohnTH, this is Firefox on iOS and not Android or Windows as you can see on right sidebar.

The Firefox for iOS is basically a shell browser so it cannot use the Extensions made for desktop Firefox or Android Firefox. Maybe in future.