I have formatted my hard drive and reinstalled my OS from Windows 7 to Windows Vista.
How can I restore my old Firefox bookmarks?
Ususally the only way you can recover files from a formatted HDD is when the HDD has seen NO or at least very little use.
If I undertstand you correctly you have installed Vista on the formatted HDD, this in all probability means that your old data has been overwritten.
If you want to try anyway search this site for using keywords like "recover file deleted format". Here is one example with some good advice.
In the future I suggest you install Xmarks to keep your bookmarks safe.
If you have formatted the drive where your bookmarks were on (and installed a new system over it already) and you did not backup the profile folder or saved the profiles in an online service such as XMarks or Firefox Sync, I'm afraid you won't be able to get your bookmarks back.