Firefox 57 is going to have a "centered" address bar just like that, but it's weird to see it in Firefox 56. I'm going to describe how to "uncenter" it in Firefox 57 and let me know if it works on yours.

Open the Customize feature using either:

• "3-bar" menu button > Customize
• right-click a blank area of the tab bar (or the "+" button) and choose Customize
• tap the Alt key to display the classic menu bar temporarily, then View > Toolbars > Customize

Do you see a kind of "spacer" to the left and right of the address bar? You can drag that down into the area of unused buttons to remove them from the main toolbar.

After using the Exit Customize button or closing that tab, is it back to normal?

If not:

This may be an experiment. Could you try this:

Type or paste about:studies in the address bar and press Enter (or Return) to load that page.

If you are enrolled in a user interface experiment, it should be listed on this page and there should be a way to uninstall/remove or to opt out.

Can you find it there?

More info about SHIELD studies and how to opt out globally if you prefer: About Studies.

"3-bar" menu button > Customize right-click a blank area of the tab bar (or the "+" button) and choose Customize

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Thanks a lot, mate !!! you solved my problem!! have a nice day!

Firefox 57 supports these spacers, so if you have them in Firefox 56 then it sounds that you have used this profile in a 57+ Firefox version.

Note that you can right-click such a spacer in Firefox 57 and click "Remove form Toolbar", so there is no need to do this in customization mode.