Vertical tabs panel stays expanded when mouse cursor leaves current display

I have Firefox configured with auto-expanding vertical tabs. Sometimes, when the mouse cursor leaves the Firefox window through the same border where the tab bar is located, the tab bar expands and stays in that state until the cursor returns to the Firefox window. This is especially annoying with two displays; if the cursor hovers over the tab bar right before moving to the second display, the tab bar remains expanded, overlapping part of the browser content.