Using 57.0b9 Win 64-bit When opening any of the menu items, such as: the hamburger menu, a right-click context menu, when you've allowed desktop notifications and one comes up from the system tray, when you sign in to a site and the Save Your Credentials pop-up is supposed to appear. In all of the above situations, Firefox will hang for about 10 seconds and then return control, or sometimes will completely hang. I haven't been able to determine any patterns with the above. When using the non-beta release of Firefox, no such issues are encountered - everything works smoothly.