Sometimes (usually this happens after switching to Firefox from other application/virtual desktop, or switching to another tab, or just after don't touching keyboard/mouse for a while on current tab) when I try to scroll (up or down) webpage using mouse wheel Firefox lags (there are small, but noticeable and annoying delay before scrolling webpage). If I quickly scroll up and down several times this lag goes away and then I can scroll on this website without any lags. This issue may happens on any websites, even simplest ones (which doesn't use background images, complex js, etc.).
Smooth scrolling is off, switching hardware acceleration on/off doesn't change anything, starting Firefox in safe mode doesn't helps too.
Opera and Chromium doesn't have this issue.
I'm using Hardened Gentoo Linux, kernel 3.10.1, nvidia-drivers 319.49, Firefox 24.0.
Hardware is fast enough: Core i7-2600K @ 4.6GHz, 8 GB RAM, GeForce GTX 560 Ti.
After googling I found several reports (many years old) which blame proprietary nvidia driver and recommend using non-100% zoom (like 101%) to make this issue less noticeable (I'm already using NoSquint with 150% default zoom). I've tried to switch to nouveau video driver, and looks like scrolling doesn't lag with it (I'm not 100% sure because I've used it only few minutes), but I need acceleration/opencl features provided only by proprietary nvidia driver so I've switched back to it.
But even if this issue is in nvidia driver, Firefox is only app which have this issue, so I'm pretty sure it can be fixed / workaround in Firefox. Maybe this issue can be fixed by changing some nvidia driver's options or Firefox config?
This is something that would need to be addressed downstream by Gentoo. Please try filing a bug with them and raising the issues in regards to Nvidia performance since they build their own Firefox package which is provided to the end user and differently from upstream.