Sorry to hear about this problem.

If you open the logins.json.corrupt in a text editor (just for viewing, editing is not recommended), does it look like an intact data file or are there runs of garbage characters or lack of an appropriate ending? As a small example (structured view on the left, text editing view on the right):

One potential issue is that the logins.json and key4.db files work together when you have a primary password set. I'm not sure either file is usable alone without it's partner. But I haven't tested...

Do you make any regular backups of your computer that might include your Firefox profile folder?

If so, you could check for a recent logins.json + key4.db in there.

Otherwise:

During system updates, Windows 10 may capture a copy of your Firefox data into a restore point. To avoid mangling other data, do NOT use system restore. Instead, try a program that lets you selectively export data out of restore points. Here are two options for that:

• ShadowExplorer: http://www.shadowexplorer.com/
• Shadow Copy View: https://www.nirsoft.net/utils/shadow_copy_view.html

Before using them, set Windows to show hidden files and folders before diving in: https://support.microsoft.com/en-us/windows/view-hidden-files-and-folders-in-windows-97fbc472-c603-9d90-91d0-1166d1d9f4b5

Then within the most recent restore point shown in the program, you can explore along this path to see whether you can find a shadow copy of your profile:

\Users\your-user-name\AppData\Roaming\Mozilla\Firefox\Profiles\your.profile

Actually, you could export out the entire Profiles folder to your desktop.

Let us know what you find and we can make further suggestions.

Thanks for the quick and details answer.

I've realized a lot of very strange things:

• logins.json.corrupt is empty, filled with 'x00'
• key4.db is almost a year old
• regular backup does not contain profile folder of Firefox (It's my big mistake)
• in the windows 10 system restore point is setted and enabled to the c: drive, but the recovery list is empty !!!

I have to check my partition backup ( it is around 6 month old) but it is not available at this moment.

Thanks if you have any additional idea my next step is reconsidering and checking my backups ..

Do you have any other profile folders under Profiles besides the one you're currently using? Or an Old Firefox Data folder on the Windows desktop?

You only have one profile folder and also do not have the "Old Firefox Data" folder on the desktop ? Are you using Sync to sync the logins to another device ?

There is no other profiles or "old FF data"

I registrated a FF - account when it became service, but I never used it. The synch function is setted off . I forget to write, but at the first starting after upgrading the FF tried to log into this account what I cancelled.