Firefox

Firefox's saved passwords are not encrypted and just copying your user profile to another computer will allow someone else to use your credentials.

If you use Chrome, do you really want Google knowing your logins?

Tldr; chromium based browers are more vulnerable to password stealing malware because they encrypt with the OS user creds, Firefox would probably avoid this with a master password, as thats the primary protection of this that password managers offer.

https://www.bleepingcomputer.com/news/security/redline-malware-shows-why-passwords-shouldnt-be-saved-in-browsers/

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, ...)