The add-on is actually not only compatible with Firefox but also with the email client Thunderbird and other Mozilla based applications such as Seamonkey, Songbird or Flock. The extension will automatically change the mouse scrolling speed in the application it was installed in.

The options of the add-on provide access to very extensive parameters that can be used to fine tune the scroll speed. Available are both basic and advanced settings.Basic mode contains settings to change the speed and step size. Speed ranges from very slow to turbo while the step size ranges from 1/10 page to 1 page or 10 pixels to 500 pixels. It might take a few tries before the ideal scroll speed settings can be found but it is very well worth it in the long run.

Holding down the alt or shift key will increase or respectively decrease the scrolling step size to allow for a bigger or smaller scrolling area. Additional options are available in the basic settings window like enabling the same scrolling speed when using the computer keyboard to scroll. The advanced settings on the other hand can be configured to change adaptive duration and steps and a fps limit. SmoothWheel is a great add-on for users who are not satisfied with the mouse scrolling behavior of Firefox or other Mozilla software programs.

The solution was first posted by Alwyn on Deviant Art. All that needs to be done is to add a line of code to the Firefox userContent.css in the chrome folder. Check the official Mozilla website that explains how to find that folder in various operating systems.

Once located open the file in a text editor and paste the following lines at the end of it. If you do not have a file that is named userContent.css yet but one that is named userContent-example.css remove the -example part of the file first and add the lines afterward. Firefox 3 has to be closed for this.

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

There is a second solution that might help some: Uninstalling and installing Flash might help as well. Use the official Adobe uninstaller and installer to do that. Make sure Firefox is closed during the uninstallation and installation.

Update: Adobe has changed the download locations and made available a Flash Player Uninstaller for 32-bit and 64-bit versions of the Adobe Flash plugin. The 32-bit Flash Player uninstaller is available here, the 64-bit version of the program uninstaller is available here.

Close all web browser windows before you run the uninstaller. You may need to restart your PC after the uninstaller has removed the Flash Player plugin on the computer.