I believe that we "capture" the spacebar keypress when we can. However if you aren't currently "focused" on the Roll20 table -- for example, if you just switched to the tab, and input is focused on the Firefox app in general -- we can't intercept it. This is just the way Javascript works, unfortunately. If you click on the game table and then hit spacebar, the scroll down should never happen, let me know if it does.
