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.
When you connect to a secure https site, then no. When you connect to unsecured http, then yes.
Firefox has its own cert store for https, and could theoretically impersonste other websites by adding their own and then sniff your traffic. But thats not how vpn providers typically work, and thered be huge backlash if they were to do that unprompted.