Wacom pan/scrolling affects videos too

I use a Wacom Intuos Pro m. I set one of the pen buttons to operate as scroll/pan. When I use scroll/pan in a page to scroll upwards or downwards, if the page hasvideo content (youtube, vmeo), the video playhead moves forward or backwards at the same tiem the page moves. In other words, page scrolling and video scrolling do not work separately, and this only happens in Firefox windows, not in Firefox mac, nor with other Windows browsers. How can I set up Firefox so the pan/scroll works as expected, namely, only having effect on the scrolling of the page? Thanks