Firefox’s free VPN will offer 50 gigabytes of monthly data, which is pretty generous for a browser-based VPN. A Mozilla account is required to make use of it, which isn’t a hardship (they’re free), but is a point of friction some may wish to know upfront.
idk about you, but when I use a VPN I want all the device’s traffic to go through it, not just the browser. So I’d always disable the built-in feature in that case.