Scroll down half the screen size in Firefox

Should be quite simple to accomplish in AutoHotkey. Download autohotkey, and use the following script with it. The script detects if you are in a input box.

#NoEnv  
SendMode Input
SetWorkingDir %A_ScriptDir% 
#IfWinActive, ahk_class MozillaWindowClass
{
    Space::
    If A_Cursor != IBeam
    {   
        Send {WheelDown 150}
    }else {
        Send {Space}
    }
    return
}             

Then adjust the amount of scrollsteps you want to scroll at {WheelDown 150} You can easily compile the script to an exefile if you want by using the Compiler that is included when you download autohotkey.

OR download this precompiled binary... http://www.megaupload.com/?d=D58JPT4U

It's just an undeveloped idea, but maybe somebody will make it real: A custom greasemonkey script, which detects space and shift-space keypresses, prevents default scroll behaviour, and programmatically scrolls half page.