I think you are talking about the toolbar, yes?

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

The blank space and flexible space in Customize hasn't been a standard part of the Firefox UI since Firefox 29 was released over two years ago. If you are were / are using an extension such as Classic Theme Restorer which 'restored' that feature to Fx29-plus, you should be able to 'grab' and move a blank space into the "Additional Tools and Features" palette. If you had that extension and it is now uninstalled, that 'feature' is gone, but the blank spaces won't disappear when the extension is removed; the underlying code is still in Firefox.

You can use the Restore Defaults button in Customize > Additional Tools and Features to reset all the Customizations.

l do use the Classic Theme Restorer l have for several months and it was only today that it started playing up.l couldnt grab hold of the blank spaces at all,yet l could grab buttons on either side of them and return them to the Additional Tools and Features palette without a problem.l fiddled around and found a fix and that was to go into the Additional Tools and Features palette and in the bottom left hand corner,l pressed the Title Bar button and found l could grab the blank spaces and remove them.

Thanks guys for the quick responses...cheers

That was very good work. Well done.