Are you using any password programs/add-ons? What about the rest of your profile? Settings, bookmarks, history, passwords . . . .

I do not believe I am using any password programs. My bookmarks are not affected, nor is my history. I haven't messed with any settings that I would notice if they changed. I have some add-ons on my phone but my phone does not have this issue, only my desktop does. As previously, it is only the usernames/passwords that get deleted.

If you have sync, and there is a problem anywhere, Shut Down Sync Immediately On All Devices to prevent the problem from spreading. Once the problem is fixed, perform the same repair on all computers/profiles before using sync again.

Look on your desktop. Do you see a folder called; Old Firefox? Look inside. Look for the folder with the latest creation date.

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