Do you have the Firefox window at full size?

Type about:customizing<enter> in the address bar. Look for Show / Hide Toolbars at the bottom left.

There is also a Restore Defaults button at the bottom middle.

Note: about:customizing is no longer present in Firefox 47+ releases.

• Press Alt or F10 to bring up the toolbar.
  Then View > Toolbars > Customize.
• Right-click on a blank spot in the toolbar and select Customize.

Thanks for the advice, but it hasn't solved the problem. I'm running FF54.01 on Win 10. Alt or F10 doesn't do anything. The only way to access the customise link is right click in tool bar, where the Menu bar option is checked; nevertheless Menu bar disappears as soon as Firefox is opened.

Try to rename/remove the xulstore.json file in the Firefox profile folder with Firefox closed.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

OK, I'm running an add-on called Menu Wizard 5.08 by Oleksandr [don't know where it came from]. When I disable it, the problem goes away. Thanks for the help. Would like to know more about this problem and Menu Wizard's disruption.

It’s very sad, but many software downloaders/ installers will trick you into installing not only their program, but other programs as well.

You have heard of the fine print in shady contracts, right? Well, some installers you need to look at the itsy bitsy teeny weeny fine print.

You are thinking you are giving the installer permission to install the program you want by using the recommended option. But if you use the Manual Option Instead, you discover all kinds of stuff that you do not even know what it is or what it does.

From now on, everyone needs to Use The Manual Option to put a stop to this.

Note that these programs can also change browser/computer settings.

Damn right Fred. It's too easy to get into that reflex action of clicking OK without thinking first.