yes, if you enable resist fingerprinting on any Firefox build it will cap refresh rate to 60hz. Mull is not doing anything special, it’s just changing about:config options by default.
you can disable resist fingerprinting in mull and regain standard refresh rate (although you lose fingerprinting protection) just as you can enable resistFingerprinting in Firefox beta or nightly and see refresh rate cap at 60.
you need to use Firefox beta, nightly, mull, or Fennec F-Droid to access about:config and from there you can search for and enable resistFingerprinting. it’s not an option in the settings.