A canary of chrome did have the ability to generate random passwords, but password management in chrome is still a pain IMO. Not sure about FF, but a quick google suggests it doesn't generate random passwords automatically.
People use other managers for many reasons: storing passwords (and other secrets) which aren't used on a site, using them on different browsers (say, Safari on the desktop and Chrome on mobile) and lack of trust on the browser's password manager.
Also, for a long time, browsers didn't save passwords with forms marked with autocomplete=off.
https://blog.lastpass.com/2014/12/introducing-auto-password-...
I works for most of the major websites (Google, Amazon, etc. I think you can look them up). And also handles multiple google accounts pretty well, even when an google account is logged in, without logging it out. And it definitely doesn't works for the majority of the websites.
(And now for the skeptical ones) I'd say use it for websites you use 2FA since any bug (or intentional backdoor) won't be successful.
PSA: If you're using LastPass for managing passwords, DONOT use their 2FA authenticator app, since now it offers an option to autofill option. Now that is the point where you're crossing into al eggs in one basket territory.