After a bit of confusion, I found out why my #Firefox was being laggy on some website despite me turning off Javascript.
Turns out I had set layers.acceleration.disabled while debugging V-Sync.
What this setting does is effectively disable hardware acceleration for a core component of page compositing; making everything laggy.
Mess with settings in a dedicated guinea profile people!
Honest question: how do you manage to navigate the modern web with JavaScript off? 99% of websites won’t work.