If you’re running Qubes, putting this in a templat’s /lib/firefox-esr/defaults/pref/autoconfig.js will disable JIT on all VMs based on that template.
// Comment to start the file.
pref("javascript.options.baselinejit", false);
pref("javascript.options.ion", false);
pref("javascript.options.wasm", false);
pref("javascript.options.asmjs", false);