Are both of your computers connected to the same Firefox Sync account? If so, you may need to disconnect the old one from the account. One option is to change your account password using your new computer, and then not update the password on the old computer. This article has the steps for that: Disable Firefox Sync on a lost phone or tablet.

Assuming you have all your information on the new computer and no longer need any data on the old computer, you can uninstall Firefox and use the option to remove your personal information. This will delete your settings folder that includes your bookmarks, passwords, and history.

You should perform a secure wipe to remove all (deleted) data stored on the hard drive as Windows only marks clusters as deleted, but doesn't remove the actual data. So deleted data can easily be restored with an undelete utility.

• http://www.thefreecountry.com/security/securedelete.shtml