From the FAQ page (https://leta.mullvad.net/faq) :

> However, Leta is useless as a service if you use the perfect non-logging VPN, a privacy focussed DNS service, a web browser that resists fingerprinting, and correlation attacks from global actors. Leta is also useless if your browser blocks all cookies, tracking pixels and other tracking technologies.

In other words everyone can benefit from it. I don't know any browser (not talking about obscure browsers like lynx) who can completely resist fingerprinting.