First, do a full (not artifact) build with the spidermonkey shell enabled. You'll need this in your .mozconfig:
ac_add_options --enable-js-shell
ac_add_options --enable-debug
This takes a while. Once it is finished you can run the shell to verify that the dis
function works:
$ ./obj-x86_64-pc-linux-gnu/dist/bin/js
js> dis(() => {})
flags: LAMBDA ARROW