You could try retrieving lost passwords by using the logins-backup.json file in the user Profile folder. Although this would need to be done sooner rather than later.

If you do not know where your Profile folder is, see here: https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Close all Firefox windows. Move logins.json to a backup folder. Copy logins-backup.json to a backup folder. Rename logins-backup.json to logins.json. Restart Firefox.

If this doesn't work you can always exit Firefox and then restore the original two files from the backup folder.

Did you try to reboot the computer? Did you only lose the logins or other data as well ?

• https://support.mozilla.org/en-US/kb/recover-user-data-missing-after-firefox-update