When you were reinstalling firefox did you uninstall all firefox install and deleted Mozilla folder?

customize/ restore should have taken care of that. here is what i do personally when a fix that should work doresnt. i keep a seconed firefox browser, firefox deelopers edition on my desktop at all times. i switch to it when a problem like that happens. i have never had that same problem when i do that, then in my spare time i find out how to fix t he problem then go back to the reg firefox it kreps me working and not frustrated. i know this isnt a fix but you have tried what i know already this is workaround

Fire Fox was Uninstalled and the folder deleted before downloading again.

Firefox 57+ auto-hides the Downloads button on the Navigation Toolbar by default. The Download button should appear on the Navigation Toolbar once you open the Customize window. If you do not see the Download icon then click the Restore Defaults button to reset the layout in case you previously have moved this button. Once you see the Download button in customize mode then left-click this button to see the current auto-hide state and toggle its setting.

Clicked Customized and saw the download button; auto hide was checked. Toggled the download button several times but still not able to restore defaults, regardless of whether or not it was checked. Dragged the button to the overflow menu and then was able to click on restore defaults - so maybe the problem is solved.

Went to customize and saw the download button. Auto hide was marked. Toggled it several times without any activation to the Restore Default button Then I dragged the toggle button down to the overflow menu and voila - the restore default button could be activated. This I did and the problem was solved. Could now see the download icon after downloading. Thanks to the responses and for your interest. jmb

You need to left-click the Download button in customize mode to change its autohide setting.

While in Customize, left clicking and changing autohide was executed but it had no affect on the 'restore default' button.

Toggling download autohide toggles this pref: browser.download.autohideButton (true/false). If you click Restore Defaults then you will reset the autohide state and the download button will disappear once again and you need to click the download button again to toggle the autohide state .