Are you asking on Mozilla support because you had previously saved the password in Firefox?

Firefox stores passwords in a pair of files. The file types and names have changed over the years, so it would depend on what version of Firefox you were running at the time you reformatted. For example:

For Firefox 4-31, look for signons.sqlite and key3.db -- you need to find both, intact, or the files are useless.

For Firefox 32 and later, look for logins.json and key3.db -- you need to find both, intact, or the files are useless.

To scrounge up those files, you would need an undelete tool like the following to have any chance of recovery, but it's hard to hold out much hope if you already put files back on your drive:

https://www.piriform.com/recuva

Is there a reason you are using a very old version of Firefox?

In the event that the passwords information is lost, you should record all information in a separate text file somewhere else on your hard drive, or written down. You can easily copy and paste if you need to do so. If you are concerned about someone else looking at that file, you can compress it using a password.