hello, firefox usually saves no data about you online - in case you're using firefox sync, all that account data is stored in an encrypted format and even mozilla doesn't have access to the plain data on the servers.

in case you're talking about where firefox is storing its data on your filesystem, please see: Profiles - Where Firefox stores your bookmarks, passwords and other user data

See also https://www.mozilla.org/privacy/

Oh yeah.. Sorry i forgot about that, thank's for the answers. Have a great day !

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