I personally use Firefox's built-in password manager and it works great for me. I use Relay to generate email masks and enter in random passwords that are saved and synced across my devices. It's been very helpful!
I use the Firefox password manager too. It's easy and convenient and I still haven't seen any reasonable claim about it being insecure, or any other reason to switch.
Am I missing out on something by not using a separate password manager?
They can get more features like categorizing and organizing your passwords, but it's just not worth it imo. I wouldn't use those features anyways. I used to use Bitwarden but I switched to FF because it's just easier and works seamlessly.
I'm surprised nobody has mentioned this already. I know Google has had a bad rep about it in the past so I moved to FF, but it's also nice having a Pixel phone and using the Google authenticator app because it works for other apps, too.