Do you mean scrolling with the mouse wheel or initiating the Firefox auto-scroll feature by pressing the scroll wheel?

In case of the latter you need to make sure that the mouse pointer isn't on an active part of the page when you click what can be difficult if there are a lot of element/images that accept click events. Once you have entered auto-scroll, you shouldn't leave it by moving the mouse up/down until you click a mouse button or press ESC.

In case of the former, you can try to move the mouse pointer to the scroll bar area to avoid scrolling frames, although Firefox should continue scrolling the page once you reach the end of that frame.

Thank you for the reply.

I am referring to initiating auto-scroll by pressing middle-click.

'...you need to make sure that the mouse pointer isn't on an active part of the page when you click what can be difficult if there are a lot of element/images that accept click events.

Yes, I am aware of this and you are describing the problem I'm trying to find a solution for: I want to have access to auto scroll at all times, regardless where the pointer is, or what is going on under it.

...I was able to find a scroll solution using AutoHotKey that allows scrolling from anywhere in the page, even while hovering over images/links within Firefox. In the example below I have it assigned to the + key on the number pad, as after some experimentation that seems to work for me the best, but this could easily be modified to any other key, or the middle mouse button instead:

NumpadAdd::
Loop
{
    If !GetKeyState("NumpadAdd","P")
        break
    MouseGetPos, ,ay
    Sleep, 100
    MouseGetPos,,by
    Send % (ay<by ? "{WheelDown " : "{WheelUp ") . round(Abs(ay-by)/15) . "}"
}
Return