The master password is for the builtin Lockwise Password Manager that uses the logins.json and key4.db files in the profile folder. It sounds that you try to login to a Firefox Account used for Sync and that is different than Lockwise. Are you using a Firefox Account Sync account on the old device and are you syncing logins ? ---- You can copy files like these with Firefox closed to a removable device and transfer them to a profile folder on another device. *bookmarks and history: places.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 (passwords) and key4.db (58+) (decryption key) for Passwords saved in the Password Manager *cert9.db (58+) for (intermediate) certificates stored in the Certificate Manager<br>if you only have cert8.db then make sure to remove an existing cert9.db *persdict.dat for words you added to the spelling 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) See also: *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile