I just noticed that the flickering of the quite long Bookmarks and History menus in the 91 release only occur in Maximized window mode, in Minimized the menus open normally, extending beyond the Firefox window bounds and the bottom of these menus appear to conflict with the Dock (appear under it?). These long menus seem to have been improved in the 91.0.1 release and the bottom of the menus appear on top of the Dock.
I just noticed that the flickering of the quite long Bookmarks and History menus in the 91 release only occur in Maximized window mode, in Minimized the menus open normally, extending beyond the Firefox window bounds and the bottom of these menus appear to conflict with the Dock (appear under it?). These long menus seem to have been improved in the 91.0.1 release and the bottom of the menus appear on top of the Dock.