This address book stores all of your contacts in the memory of your web browser. This means that if you enter contacts into Firefox at your work computer, you can only access your contacts in the same browser on the same computer. However, you can easily export your data and import it into a different browser. Just hit the Export button and copy the data that populates the export box. Save it in a text file. When you're in a different browser (or want to replace the data in your current browser), paste that data into the Import box and hit the button.
Contact Jeffrey Kishner at firstname.lastname@example.org or kishner if you would like to report an issue.