Windows only: If you are accustomed to scrolling quickly through long documents, there's a secret Firefox 3.6 config setting that allows you to enable acceleration, which will scroll more quickly based on how many times you've scrolled.
To tweak the acceleration for yourself, head into Firefox's about:config page, and then filter by mousewheel.acceleration to find the two settings that apply — the mousewheel.acceleration.start setting actually enables acceleration by setting the number of times you need to scroll your mouse before the acceleration kicks in, while the mousewheel.acceleration.factor setting specifies how much acceleration to apply.
The new mouse wheel acceleration feature is disabled by default in Firefox 3.6 because it can conflict with your mouse drivers, especially if you've enabled faster scrolling in the Mouse panel in Control Panel, but you can tweak the setting either way. It's not a setting that everybody will want to apply, but if you're unhappy with the current Firefox scrolling, you can play around with the settings to figure out your preferred level of acceleration. It only works in Firefox 3.6 on Windows, and make sure to hit the link for a full explanation on how it works.