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!
@Scrollone
Ah, I was being a bit unclear. It was lagging and I suspected JS to be the cause, so I temporarily turned it off.
To my surprise however, it was still laggy despite no JS being involved.