Hey! So sorry to hear about this :( Could you go to about:config and then search up the preference browser.backup.backup-debug-info and share what it's set to?

{"lastBackupAttempt":1777034400,"errorCode":{"name":"BackupError","stack":"#prepareStagingFolder@resource:///modules/backup/BackupService.sys.mjs:1976:25\n","unremovableContents":["C:\\Users\\User\\AppData\\Roaming\\Mozilla\\Firefox\\Profiles\\4yhq4lxf.default-1533239578278\\backups\\snapshots\\2026-03-26T10-15-28Z.zip","C:\\Users\\User\\AppData\\Roaming\\Mozilla\\Firefox\\Profiles\\4yhq4lxf.default-1533239578278\\backups\\snapshots\\archive.html","C:\\Users\\User\\AppData\\Roaming\\Mozilla\\Firefox\\Profiles\\4yhq4lxf.default-1533239578278\\backups\\snapshots\\staging-0","C:\\Users\\User\\AppData\\Roaming\\Mozilla\\Firefox\\Profiles\\4yhq4lxf.default-1533239578278\\backups\\snapshots\\staging-1","C:\\Users\\User\\AppData\\Roaming\\Mozilla\\Firefox\\Profiles\\4yhq4lxf.default-1533239578278\\backups\\snapshots\\staging-2","C:\\Users\\User\\AppData\\Roaming\\Mozilla\\Firefox\\Profiles\\4yhq4lxf.default-1533239578278\\backups\\snapshots\\staging-3"]},"lastRunStep":5}

Looks like a problem with #prepareStagingFolder

• https://searchfox.org/firefox-release/source/browser/components/backup/BackupService.sys.mjs#1966

Are there messages in the Browser Console related to the backup?

• https://firefox-source-docs.mozilla.org/devtools-user/browser_console/

Decoded:

 lastBackupAttempt:1777034400; (2026-04-24T12:40:00.000Z)
 errorCode:
  name:BackupError;
  stack:#prepareStagingFolder@resource:///modules/backup/BackupService.sys.mjs:1976:25\n;
  unremovableContents:
   0:C:\Users\User\AppData\Roaming\Mozilla\Firefox\Profiles\4yhq4lxf.default-1533239578278\backups\snapshots\2026-03-26T10-15-28Z.zip;
   1:C:\Users\User\AppData\Roaming\Mozilla\Firefox\Profiles\4yhq4lxf.default-1533239578278\backups\snapshots\archive.html;
   2:C:\Users\User\AppData\Roaming\Mozilla\Firefox\Profiles\4yhq4lxf.default-1533239578278\backups\snapshots\staging-0;
   3:C:\Users\User\AppData\Roaming\Mozilla\Firefox\Profiles\4yhq4lxf.default-1533239578278\backups\snapshots\staging-1;
   4:C:\Users\User\AppData\Roaming\Mozilla\Firefox\Profiles\4yhq4lxf.default-1533239578278\backups\snapshots\staging-2;
   5:C:\Users\User\AppData\Roaming\Mozilla\Firefox\Profiles\4yhq4lxf.default-1533239578278\backups\snapshots\staging-3;
 lastRunStep:5;

Browser Console

Maybe check the write permissions of the snapshots folder with the path in the screenshot as Firefox isn't able to remove previously saved snapshots.

It could also be possible that your security software keeps an handle on files.

Reboot the computer into Windows Safe Mode with network support to see if that has effect.

• https://www.bleepingcomputer.com/tuto.../how-to-start-windows-in-safe-mode/
• https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/

maybe just delete these files?

my antivirus is always turned off, except in a few cases.

There's also a message (the first one) that Firefox can't write to "C:\Users\User\Documents\Restore Firefox", which would be critical for creating a backup. So, while you can remove the mentioned folders/files manually, that alone may not be enough to fix this.

after deleting the files from the "snapshots" folder, the backup went without error. but when I started the backup again, i got the error again, then i just deleted the "backups" folder – now everything works.

but its strange that such characters appear – apparently there's a problem with cyrillic.

The release notes for the latest Firefox version, 150.0.2, say "Fixed an issue that could prevent Firefox’s local backup feature from completing successfully", if this is related.