Firefox

On top of what has been said on lower security, this is also much worse in terms of privacy. You are giving up your credentials to Google/Mozilla.

Also they do not encrypt (when they do at all)website URLs, only the secret parts (passwords) so this is a downside as well. Anyone getting access (or Google/Mozilla) of the encrypted vault knows what apps/sites you have accounts on. Some password managers do encrypt everything

But yes, primarily it is way easier to steal passwords from a browser, especially when it's synced across many devices including some with lower security (a phone with just a pin, a phone lent to other people, a computer or tablet or phone let open to anyone to change music on Spotify, ...)