I'm using Firefox on Arch Linux with XFCE desktop. Since the firefox update to 48.0, my window positions are not correctly restored at restart or close / reopen. I have created two screenshots. One before closing, one after reopening. For better view, I have cropped the screenshots and combined them to one picture. As you can see, the offset matches the height of my taskbar, placed at the top border of the screen. Every window, except the one that's already at the top border, is affected. If I close and reopen firefox again, the windows will also shift again. If I repeat this some more times, all windows are at the top border. I've already tried to delete xulstore.json, as described in https://support.mozilla.org/de/questions/1103841 but no change. Disabling my AddOns also did not help. When I open a profile that has not yet been opened since the update, the positions are correct. After restarting, the window positions are shifted. When I restore the profile from my backup, the positions are correct again. That's why I think the error occurs during saving of hte window positions, not during restoring. It looks like firefox does no more take the taskbar into account and uses the absolute top of the screen instead of the area below the task bar. Is there some trick, or could this be a new bug ?