You may have removed the button(s) from the Navigation Toolbar by accident.

If you right-click a toolbar item in normal (non-customize) mode then you can remove this item from the toolbar.

• "Remove from Toolbar" will remove this item from the Navigation Toolbar and moves it to the Customize palette ("3-bar" Firefox menu button > Customize)
• "Move to Menu" will move this item to the "3-bar" Firefox menu button drop-down list
• "Move to Toolbar" in the "3-bar" Firefox menu button drop-down list will move this item to the toolbar

Open the Customize window and set which toolbar items to display.

• "3-bar" Firefox menu button > Customize

• if missing items are in the Customize 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 Customize palette then click the Restore Defaults button to restore the default toolbar setup

See:

• https://support.mozilla.org/kb/customize-firefox-controls-buttons-and-toolbars

Bonhomme said

They have disappeared and I can't find how to resurrect them. Ctrl+ and Ctrl- is available but not so convenient.

Now restored from 3 bar menu. Thanks very much.