I'm using FF 77.0.1 (64-bit) [Mozilla Firefox for Ubuntu canonical- 1.0, but the problem has been present for the last 2-3 months over those versions current. When I open a folder on my bookmarks toolbar, the menu pops up on another display. If I just move my mouse down, as if the menu were immediately below the folder link, the corresponding rows on the menu on the other display are highlighted. However, clicking at that point does not select that menu entry. I must move my mouse over to where the menu is displayed, all the way being careful to not move outside of the Y dimension range of that menu (or it vanishes). This does not happen on all folders on the menu, but the ones it happens on vary from day to day. Prior to the version of FF that I am on, it would only happen once to a given menu and then it would start opening the menu where it should. I currently have 15 folders showing on the toolbar and 5 of them consistently misbehave. I'm on Ubuntu Linux 18.04 using the Gnome desktop/window manager (not Wayland). I am using a Dell Laptop XPS-13, connected to a Dell DisplayPort dock which drives two external displays, on which I usually do all my work. If I drop back to just the laptop, all the folder menus display properly. In addition, with the dock connected and the two external displays, if I move the FF window to the laptop's display, the menus display properly. However, in my use of multiple applications, no application besides FF exhibits this problem. If I open a private session, the problem is not exhibited. Adding a screenshot example. I had to crop it on the bottom side to fit in the 1024 Byte limit for attachments. On the bottom, you see a black area on the left, which is where there is no monitor. On the right is a portion of my laptop's display (cropped on the bottom). On the top are the two displays, and the portion just above the laptop display is my right display, and left is left.