Has the problem just started to happen? If so then something has changed to cause it so have you changed anything or installed Win10 updates? Sometimes Windows updates cause problems due to sudden incompatibility with installed programs. Clutching at straws here but have you looked at "View", "Toolbars" and then "Bookmarks Toolbar" to see that it is ticked? Maybe try uninstalling and reinstalling FF?
Gamescom 2018: Announcements, livestreams and exhibitors