Hi, I'm not sure what reinstalling Win10 does, but your best chance is to open bookmarks > Show All Bookmarks > Import and Backup > Restore, and see if you have a backup there. If not, please see - *[[Profiles - Where Firefox stores your bookmarks, passwords and other user data]]. *[[Recovering important data from an old profile]]. ''If your question is resolved by this or another answer, please take a minute to let us know. Thank you!''

You can also use backup files if you have any. Was the disk 'cleaned?' Look for a folder like WindowsOld or some such.