I guess this question is about Firefox VPN. Naming is confusing. Yes, it's free.

Firefox VPN is a built-in Firefox feature that adds privacy by routing your browser traffic through a secure proxy server and masking your IP address. The feature includes a monthly data limit of 50 GB. Firefox will notify you when you are approaching this limit with a prompt in the browser.

Unlike paid Mozilla VPN, which protects your entire device, the built-in VPN only masks traffic that originates in Firefox. It's free to use and doesn’t require a subscription.

If it's part of my Firefox account, that's fine,

Btw the Firefox Accounts has been renamed as Mozilla Accounts since November 1, 2023 to reflect that it is being used for multiple things including this support.mozilla.org site and not just for Firefox Sync. https://support.mozilla.org/en-US/kb/access-mozilla-services-firefox-account#w_what-services-can-i-use-with-a-mozilla-account

More info about the built-in VPN in desktop Firefox is at https://support.mozilla.org/en-US/kb/built-in-vpn