Many Firefox 3 for Windows users are complaining that when they scroll on web pages it can be very slow and choppy. There is a way to fix this problem though.

1. Close Firefox 3 and navigate to the chrome folder. The location may vary. Mine was located at:

C:\Documents and Settings\username\Application Data\Mozilla\Firefox\Profiles\randomvalue\chrome

2. Right click the file userContent-example.css and select Rename.

3. Take out the -example to give the file a name of userContent.css.

4. Open userContent.css in Notepad.

5. Copy and paste the following code at the end of the file:

/* Smooth Scrolling: Disable Fixed Background Images */
body {
background-attachment: scroll !important;
}

6. Save and close Notepad.

Now give Firefox 3 a shot. Hopefully the scrolling is quicker and smoother.

Source: Fix Firefox 3 Choppy Scrolling Via UserContent.css