More information would help. For instance:

• Have you completely reinstalled Windows & and Firefox from scratch on to a brand new Hard Drive ?
• What happened to the old Hard Drive is it still working and is it installed in the same computer ?
• Is the previous Firefox the one that was last in use.
  • and is that fairly recently? (About how long ago,and do you know the Firefox version ? )

First, let's call them Bookmarks. :-)

Do you have your old hard drive connected, or a backup of it? Your Firefox bookmarks are stored in a personal settings folder known as your Firefox profile folder. The location varies depending on whether you had Windows XP or Windows Vista/7 on the old hard drive.

XP - C:\Documents and Settings\username\Application Data\Mozilla\Firefox\Profiles

Vista/7 - C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles\

Your first profile folder names contain random letter/numbers then -default, but you might find multiple profile folders. In that case, the most recently updated probably is the one you want. Look inside for a folder named bookmarkbackups.

Let's assume you got that far and you have a number of dated files with a .json extension. You can restore those backups to your new Firefox. This article describes how (you will use the Choose File... option to get at that old file): Restore bookmarks from backup or move them to another computer.

Any luck?