Hi, it looks like it may be a 'Flexible Space.' Open the 3-bar menu > Customize, and see if you can drag it off the toolbar to the palette below, then click on Done: More Customize Firefox controls, buttons and toolbars.

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You should be able to right-click those flexible spacer items in normal mode to remove them from the toolbar. The two spacer items at the left end and at the right end of the urlbar container have been there for quite some time.