Digg released an official Firefox 3 extension that adds a digg toolbar to the Firefox browser, the tool will let you know if the page you are visiting is already submitted to digg, if it is submitted it will show digg count and the comment count. If the page is not submitted you will see a submit button for you to submit the story.
Download the extension from here, and install it, first time it will ask you to enter your digg username and digg topics of interest to you.
The toolbar also alerts you to when new stories become popular on digg by showing the story details as notifications in the bottom of the browser. Using the toolbar you can also follows your friends diggs.