Open a webpage with a scroll-bar.
Hold down the left mouse button on the scroll widget (whatever it's called.)
Move the mouse off the Firefox window, to the right.
Right-click (while still holding the left mouse button.)
Let go of the left mouse button.
Move the mouse back into the Firefox window.
Move mouse up and down to scroll without having to click.
The "mouse-up" event will have been lost, and you'll still be in control of the scroll bar, without having to hold any buttons down. Click the left mouse button again to generate a new mouse-up event and stop scrolling.
It's a bug, obviously, but it's a kinda neat and useful one.