Firefox stores usernames and password, among other stuff, in profiles. See https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles to find the location of the "Root Directory" of your profile.

Before installing Firefox on a new system, copy the existing profile directory from the old system to the same place on the new system (change the username in the path, if it is different from you old system). A new Firefox installation will then continue to use that one instead of creating a blank profile.

Obviously, if you have lost the old setup completely, there is no way to get that data back. As the saying goes: Save early, save often.

Thanks but I cannot copy files before loading the new system as it has already reset (Windows 10) I have found a file of the old passwords but cannot see how to load them to the new system. A minor point is that I use UK English not US.

Do you have a "C:\Windows.old" folder with files from your previous Windows installation ?

You can copy certain files with Firefox closed to the current profile folder to transfer or recover personal data. Note that best is to avoid copying a full profile folder.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data\
• C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>

• bookmarks and history: places.sqlite
• favicons: favicons.sqlite
• bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder
• cookies.sqlite for the Cookies
• formhistory.sqlite for saved autocomplete Form Data
• logins.json (encrypted logins;32+) and key4.db (decryption key;58+) for Passwords saved in the Password Manager
  key3.db support ended in 73+; to use key3.db in 58-72, make sure to remove key4.db
• cert9.db (58+) for (intermediate) certificates stored in the Certificate Manager
• persdict.dat for words added to the spell checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences)
• sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)

I don’t have a windows old file and I am afraid that do not understand the rest of your message. I need a step by step instruction on how to load my old logins onto the currently running firefox programme