It's a lot more laggy and jerky in GNOME/X, even with GL-render on. Is this normal with nVidia cards or should I try tweaking some settings?

Have you installed the Nvidia proprietary driver? I have 3 machines with Nvidia cards ranging from a 6600 to a gt220 and I don't have any issues. I install the Nvidia proprietary drivers as soon as I am able though, so if you are using different drivers I can't speak as to what's normal or not.

I'm using the RPM fusion drivers. Those are proprietary, right?

The fusion drivers are nVidia drivers, but they've been packaged, which means they are somewhat generic in how they are compiled. I always download my drivers from nVidia and compile them for my particular machine, and that always works well.

You didn't specify your hardware, or your Linux distro or the version of Firefox you are running on Linux, or the version of FF you are running on Windows. You also didn't specify whether your scrolling issues are on the same machine with a different OS loaded.

Lacking all of that information, as far as I can tell you are comparing apples and oranges.

I will say that I recently upgraded my system to a quad core phenom system. Prior to my upgrade, I was running an Athlon 2700 with KDE4, 2 gigs of RAM, and Firefox 3.5.something. I had lots of problems with updating screen, jerky and laggy scrolling, and so forth.

Part of my problem was FF. It was getting rather slow prior to 3.6...with 3.6 a lot of improvement were made. Part of my problem was KDE4...it is a very heavy desktop system and needs a lot of horsepower. But my basic problem was that it was just time to upgrade my hardware unless I wanted to simplify my system.

I didn't want to simplify my system; quite the contrary. So I upgraded. As I say, quad core 3.2 GHz 64 bit processor to replace the old single core 2166 GHz 32 bit processor, and an nVidia GT240 video card.

Performance is excellent. Smooth, no lags, no issues. I'm quite pleased.

So, you may be having a problem that shows when you use FF, but that may just be because FF has become a bit of a pig and you are running a fairly heavy system behind it, and you are a bit lacking in horsepower. Keep an eye on your CPU usage; if you are sitting at or close to 100% for extended periods, then you have a problem

Or, it COULD be your drivers. You might try downloading directly from nVidia and compiling specifically for your machine.

How does it do when you try to play videos? Especially high-def videos?

Do you have lots of tabs open? Do you keep pages with Flash elements open while using another tab? These factors can lead to firefox being laggy.

Ah, sorry. I'm using a GeForce 210. I actually tested this on windows. Same number of tabs and same web pages. It's slower in X compared to Windows. Hi-Def videos play absolutely fantastic in Linux, I've got Monsters vs Aliens. WMV, 1280x544, 5800kbps, with 768 kbps 48000 Hz 6-channel audio, going via smplayer. I'd say smooth playback with that is a good thing in linux when I'm also burning a blu-Ray DVD and using ffmpeg2theora all at the same time.

Firefox lags with nothing running.