Check that the Bookmarks Toolbar is visible and that the "Bookmarks Toolbar items" is still placed on the Bookmarks Toolbar.
If the menu bar is hidden then press F10 or hold down the Alt key, that should make the "Menu Bar" appear.

Make sure that toolbars like the "Navigation Toolbar" and the "Bookmarks Toolbar" are visible: "View > Toolbars"

• Open the Customize window via "View > Toolbars > Customize" or via "Firefox > Options > Toolbar Layout" (Linux, Windows)
• Check that the "Bookmarks Toolbar items" is on the Bookmarks Toolbar
• If the "Bookmarks Toolbar items" is not on the Bookmarks Toolbar then drag it back from the toolbar palette in the customize window to the Bookmarks Toolbar
• If you do not see the "Bookmarks Toolbar items" or other items that are missing in the toolbar palette or on a toolbar then click the "Restore Default Set" button

• http://kb.mozillazine.org/Toolbar_customization
• https://support.mozilla.com/kb/Back+and+forward+or+other+toolbar+items+are+missing

Thank you very much.

I performed all the remedies detailed above to restore the Favorites button to the Bookmarks Toolbar, but to no avail. The Favorites button is still missing from my Bookmarks Toolbar.

It disappeared last evening when some cosmetic changes appeared on my Navigation bar.

I earlier had imported my Favorites from Internet Explorer, but they are buried deep in the Bookmarks menu, and do not permit of adding a new Favorite -- so aren't practical to use.

If I could sync my Favorites to my Bookmarks, I could finally abandon the Favorites in favor of the Bookmarks -- but apparently Firefox does not allow for such possibility.

Disappearance of the Favorites button from the Bookmarks Toolbar makes Firefox all but unusable for me -- unless I can find a solution.

I have no add-ons, and no new plugins that might be causing this problem.

I solved the problem, per a suggestion from the Support Forum: I simply exported my Favorites, as an HTML file, from Internet Explorer 8.0 to the Desktop. Then I imported the file to Firefox by selecting Import Bookmarks from HTML... under the Import and Backup button in the Library toolbar.

Finally, I closed and reopened Firefox. There was my Favorites button, once again on the Bookmarks Toolbar.