I am running Windows 10 and Firefox is not my main browser. Also I did not assign a specific folder to save my downloads to. In the past, when I downloaded a pdf, it would ask how I would like to open it. I would select Firefox and it would open fine. Unfortunately, when I selected to always do this, it would continually open blank tabs instead of opening the pdf. I would have to completely close Firefox to get it to stop. No problems when restarting. If I restore the previous session, there isn't a problem until I load one of those tabs again. Then It starts again.