You can enable or disable SpiderMonkey's various JITs in Firefox's about:config prefs. Try disabling all the JIT prefs and running some JS benchmarks to measure interpreter-only mode. :) Each JIT level is roughly 10x faster on my machine.
javascript.options.baselinejit controls the first-pass JIT.
javascript.options.ion controls the IonMonkey optimizing JIT.
javascript.options.asmjs controls the OdinMonkey asm.js JIT.
javascript.options.wasm and javascript.options.wasm_baselinejit controls new WASM JITs.
Yes last I checked you could still build firefox on platforms they didn't have a JIT for, so it certainly must be possible. Sadly, SpiderMoney isn't as commonly used for embedding compared to V8. Maybe Spidernode will take off in the node community and make it a more popular option... we'll see.
From watching the video it seems that Ignition is ~2x slower than JIT which sounds like great performance for a non-JIT'ed implementation.
javascript.options.baselinejit controls the first-pass JIT.
javascript.options.ion controls the IonMonkey optimizing JIT.
javascript.options.asmjs controls the OdinMonkey asm.js JIT.
javascript.options.wasm and javascript.options.wasm_baselinejit controls new WASM JITs.