The Back/Forward button use right-click context menu (or hold click for 1 sec) instead of a drop down button. Context menu was always there, the Mac like holding was added in 4.0

Did you look at the right side of the Navigation Toolbar

• "ALT+V" (view menu) > Toolbars > (check) Navigation Toolbar

If only the buttons are missing look for them, in

• View > Toolbars > Customize

Otherwise could be an extension , see if if works with extensions disabled

• Help ("Ctrl+H") > Start Firefox with add0ons disabled

Are you using a third-party Firefox theme or the default theme? in Tools->Addons->Themes as that could be a cause also.