If you use the Customize feature, can you confirm that your Bookmarks Toolbar has its usual contents?

View menu > Toolbars > Customize

(Screen shot attached for comparison)

If the bar does not look right in this view, the Customize dialog also has a reset button to returns the toolbars to their original setup you could try.

If that doesn't help, can you think of any add-ons which might alter your toolbar area or bookmarks specifically? You can review here:

Tools menu > Add-ons > Extensions category
Tools menu > Add-ons > Appearance category

If you use the Stylish extension, some of its rules can modify the toolbar area. In that case, check the UserStyles category.

Firefox allows you to change the layout of the toolbar area by creating rules in a file named userChrome.css. If that doesn't sound familiar, it probably is not applicable to you. However, it's easy to check for such a file.

Help > Troubleshooting Information > "Show in Finder" button

If you see a folder named chrome, open it and check inside for userChrome.css. If you do not see a folder named chrome, then this is not an issue for you.

If your toolbars resist fixing through the interface, you also can rename the file that stores toolbar customizations so Firefox generates a fresh one. Here's how:

Help > Troubleshooting Information > "Show in Finder " button

Switch back to Firefox and Exit

Pause while Firefox finishes its cleanup, then rename localstore.rdf to something like localstore.old

Restart Firefox. Hopefully your toolbars are back to normal.

Make sure that toolbars like the "Navigation Toolbar" and the "Bookmarks Toolbar" are visible.

• Firefox menu button > Options
• View > Toolbars (press F10 to display the menu bar)
• Right-click empty toolbar area

Use Toolbar Layout (Customize) to open the Customize window and set which toolbar items to display.

• check that "Bookmarks Toolbar items" is on the Bookmarks Toolbar
• if "Bookmarks Toolbar items" is not on the Bookmarks Toolbar then drag it back from the toolbar palette into the Customize window to the Bookmarks Toolbar

• if missing items are in the toolbar palette then drag them back from the Customize window on the toolbar
• if you do not see an item on a toolbar and in the toolbar palette then click the "Restore Default Set" button to restore the default toolbar setup

• https://support.mozilla.org/kb/How+to+customize+the+toolbar
• https://support.mozilla.org/kb/Back+and+forward+or+other+toolbar+items+are+missing