As a general rule, browser based password storage is less secure than a standalone offering. While convenient, Firefox loads the cipher into memory. and stores passwords in a local file (logins.json) encrypted with 3DES (older versions) or AES (newer), using a key derived from an optional primary password. Without a primary password, Firefox uses a blank key, making it trivially decryptable. Even with one, decryption occurs locally but lacks the layered, zero-knowledge design of something like Bitwarden. This makes Firefox stored passwords more vulnerable to something like a virus outbreak on your computer, which can access your Firefox stored passwords.
This is how I understand it. If someone has better intel, or if I need schooled up, do share.