Hi Luke, you are correct that Firefox does not use a CSV file for saved logins except as an export format.

Where did you find the older data? One thing you might want to check for is additional profiles. The first section of the following article describes using the About Profiles page for that: Recover lost or missing Bookmarks. If you can resume use of the older profile, hopefully that is now not too far behind.

If it is necessary to persevere with the new profile, you would be replacing the (relatively empty) key4.db and logins.json files with the older ones, while Firefox is not running.

I'm the only user on this machine, so I never set up a proper Firefox account (definitely a regret now something's gone wrong) but the new install doesn't appear to have overwritten the corrupted one so there's a generic-named profile (which is to say just a random alpha-numeric string) containing files older than today's date that I found the key4.db and logins.json in. I've backed them up just in case Windows gets over-zealous about cleaning up.

Do you think I could just copy/paste the two files from the old profile to the clean install and overwrite the new instances? Or is the password data encrypted to a specific install or some security measure of that nature?

logins.json and key4.db are a pair (the key in key4.db is used to encrypt the passwords in logins.json) so they should work together in a new profile if they worked in the old profile.

Is the other profile listed on the About Profiles page -- to open that, type or paste about:profiles in the address bar and press Enter to open it. You can use "Launch profile in new window" on that older profile to see whether the passwords in there are the ones you are looking for. (Assuming it's on the list.)

It is on there! That's interesting. Okay, I'll handle it in-app rather than trying to brute force it by overwriting files. Thanks a bunch! That was a frustration I could have ended up burning a good few hours on.