This is going to sound nuts so I had to make a video... http://youtu.be/Difz6EccmF8 The monitor is a Dell UP3214Q connected to one of my NVIDIA graphic cards via mDP 1.2. The first half of the video the monitor is set to SST (single-stream transport) mode. The second half of the video the monitor is set to MST mode (60 Hz). While in SST mode the drop-down menu for the Firefox bookmark "star" and various other add-on drop-down menus work just fine. Click on the icon and the drop-down menus are displayed. However, when I switch to MST mode the drop-down menus shown in the first half of the video will not be displayed correctly. In fact it appears that they're simply not being rendered at all but. However, if you look closely (you might not be able to see it in the video) the drop-downs are actually working, they're just opening and closing in a fraction of a second. This only occurs in MST mode. I've started Firefox in SAFE MODE which fixes the Firefox bookmark "star" issue. However, as all add-ons are disabled in this mode I'm unable to test them. What's odd is that if I manually disable all of my addon-ons in Firefox the issue is not resolved. SAFE MODE must be doing something else but what? I've never had this issue prior to going to a 4K monitor with MST. Thank you.