Mozilla should be careful suggesting the use of VPNs (without providing a disclaimer) since most of the well-marketed ones are, for all intents and purposes, MiTM blackboxes [0], owned by state-actors in some cases [1].
For people looking to make an informed decision, take a look at https://thatoneprivacysite.net/#detailed-vpn-comparison or setup one yourself https://github.com/trailofbits/algo
If folks do inadvertently flock to free VPNs, the situation would get way worse (tracking now not just limited to HTTPS, but everything above layer-2).