If you can autofill passwords without authenticating in some way, they are probably either stored in plaintext, or encrypted with a key that is stored in plaintext. Cause, like, how is it supposed to magically encrypt it.

I believe Firefox (and forks) only encrypt if you have set a master password.

Firefox and chromium browsers also store them in plain text. I know because I literally copied them from a file when setting up my password manager.