Follow steps 1 and 2 here: https://firefox-source-docs.mozilla.org/setup/linux_build.html#building-firefox-on-linux
Then set up a debug mozconfig and build using it as described here: https://firefox-source-docs.mozilla.org/js/build.html#developer-debug-build
Once you're done you should have a mozilla-unified/obj-debug-x86_64-pc-linux-gnu/dist/bin/js
. Building from source is necessary to get the ability to disassemble native code.