Snowy, I used to have the exact same problem. Even now, there's a slight hint of it. A stuttering. Vampothika - I really think it's more Firefox specific, because going over to Pale Moon (12.0 x64) solved that, and many of the extensions are cross compatible.
Why it happens, I haven't a clue. It could well be skin or WB related... but I really think it's something in Firefox too.
I agree with the "newer' vs. 'older' skin difference you saw... Noir doesn't have this problem whereas 'Stealth Drone' does. I don't think there's anything wrong with the skin... but I bet Jeff could get to the bottom of it.