I don't know if anything changed in that area, but in the past you could disable that completely via "enterprise policy". See https://github.com/mozilla/policy-templates for a terse description. E.g. NixOS sets "DisableAppUpdate" for FF built by its build system.
@ErictheCerise @robryk Same thoughts but regarding disabling #JIT.
@robryk
I'm aware, and I'll probably implement it sometime in the next 5-8 "no I don't want to update now" button-clicks because, let's face it, I'm probably not going to hear back from that FF dev.
Nevertheless, a hack-ish work-around that requires me to create a json file from scratch does not, in my book, qualify as a solution.