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.
The free option is limited to a certain amount of GB. Mozilla can upsell an unlimited version in the future. Likely the reason they don’t do that right from the start, is that their VPN network is completely new and it’s hard to judge the network capacity needed.