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.
It doesn’t seem like it, or at least there’s zero evidence I’ve seen that this is the case. As the linked OMG Ubuntu article speculates, probably the main benefit financially is making users more likely to sign up to their paid VPN.
Aside: Based on their blog post, the service seems like a proxy rather than a VPN.