Yes you can. There's a folder in your Application Data folder for Firefox that contains the default profile in it. You can then copy that folder and paste it into your existing one. It saves your stuff like bookmarks, and even themes and add-ons too i think.
Here's the directory path for Windows XP, just change Account Name Here to your account name on the computer:
C:\Documents and Settings\ACCOUNT NAME HERE\Application Data\Mozilla\Firefox