Not only will it add some thumbnails as shortcuts, but you will be able to customize them. Choose the number of websites that will appear on your Speed Dial tab and even group them in different tabs.
Speed Dial substitutes the white background of a blank tab and features shortcuts for your most used websites.
It will take into account the number of times you visit the websites and it will show the most visited ones. You'll see a preview of those websites and you'll be able to access them just clicking them. If you want to add a certain website to SpeedDial, right-click it and assign it a box in the showcase.
From now on, firefox users will be able to enjoy SpeedDial as they used to do when they used Opera.