You say you formatted then reinstalled Windows 10 and Firefox...? If so if you did not save the Profile Folder or do the fyi below then they are gone.

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Fyi for future:

• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer
• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Please let us know if this solved your issue or if need further assistance.

Didn't you backup any of your personal data?

You can check if you have an C:\Windows.old folder with your previous data. See also:

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

If there aren't any backups available then I'm afraid that your bookmarks and other personal data are lost.

See also:

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles