This article describes how to restore Firefox's default toolbar configuration if the Back button, Forward button, or other toolbar buttons are missing.
If you can't see any of the toolbar buttons (Back, Forward, Reload, Stop, Home) or the Location bar, you may have the Navigation Toolbar hidden. Click the Firefox View menu, select the Toolbars sub-menu. If there is not a check mark next to Navigation Toolbar, click on it to put a check mark by it.
If showing the Navigation Toolbar does not restore your missing buttons, continue to the next section.
To put the toolbar buttons back into their default places:

The default toolbar buttons should now appear in their default locations. If you would like to rearrange your toolbar items, see How to customize the toolbar. If your toolbar buttons are still missing, continue to the next section.
If the above methods fail to restore your toolbar buttons:
First, start Firefox's Safe Mode:
/Applications/Firefox.app/Contents/MacOS/firefox -safe-mode
/path/to/firefox/firefox -safe-mode
firefox -safe-mode
The default toolbar buttons should now appear in their default locations.
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