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.

Were you using any password add-ons?

Thank you for this but the horse has bolted, and site specific passwords have not been held separately, we trusted Firefox to remember them for us over many updates.

Where can I find the file which holds the information, or has the update deleted it completely?

The passwords are stored in two files. key3.db and logins.json Both are needed.

Look on your desktop. Do you see a folder called; Old Firefox?

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

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

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible