Myke, sometimes missing passwords can be retrieved using the logins-backup.json file in the Profile folder on the hard drive. 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

To attempt password recovery:

1. open your Profile folder on the hard drive
2. close Firefox if it is running
3. move logins.json to a backup folder
4. copy logins-backup.json to a backup folder
5. in the profile folder, rename logins-backup.json to logins.json
6. restart Firefox

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

Thanks TechHorse for your fast and detailled answer. Sadly, this didn't help with my issue - perhaps I was too late (I already had recovered some passwords)!?