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.
I think it would be better to compare this offer to well-known VPN providers instead of all VPN providers, since the sketchiest ones tend to have the lowest prices. The two reputable ones I can think of, Proton and Mullvad, both cost over $5/month. But cost is only half of the picture: They’ve also earned their reputation through a lot of time, effort, audits, even government raids.
Regardless, you have some good points. Let’s take for granted that Mozilla will not attempt to share or sell user data with this free service, that it’s all above-board (a fair assumption): They still have to build their reputation from zero.