Type about:customizing<enter> in the address bar. In the new window, look for the icon. When you find it, hold down the left button on it, and move it to where you want it to be. You can move any of the icons you want from here, but; some icons are locked in place.

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 suggestiions but neither of these work, in that nothing happens or an error message pops up. In addition, before getting your message I right clicked on the navigation bar and clicked customise, and tabs came up where I found the option to move the tabs, hurray, but if I try to do that now, nothing happens on customise but a blank page. I think something is seriously wrong with this Firefox update. How do I roll back to an earlier one?

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

Thanks for the guidance. Under safe mode I could put the tabs back at the top, but the tabs still don't open as the ones I used last time, and if I restore my chosen skin, clicking tools - customise - opens a blank page. Maybe the skin has a problem with the latest Firefox - I wil ltry another - I wish they would make it possible to use the cat one again as that was perfect.

Thanks - the problem after going through your suggestions seems to have been the update on the skin I was using (walnut) walnut 2 lost the lizard and it appears lots of its functions, so I will have to use something else until Firefox finally makes itself compatible with the excellent red cat blue and green flavour ones where every icon is related to the theme.