New greyed-out toolbar cannot be removed (SOLVED)

[vtmozilla]

I've just noticed the appearance of a new greyed-out toolbar on my Seamonkey 2.53.16 64 bit on Linux. It displaced my Navigation Toolbar, which I had to restore. It also seems to be non-removable. I tried right-clicking on it but no menu appears giving an option to remove.

Here's a screenshot of the top of this page -- the red arrow points to the unknown toolbar.

https://ibb.co/8BB69mz

What is this, and how can I get rid of it?

Is there any way to delete a toolbar other than right clicking on it? I can't find anything in Edit>Preferences that applies.

Additional info:
1.) tried safe mode, it was no longer present. But returned after restarting without safe mode
2.) tried disabling addons (I have only three) one by one and restarting after each removal. No change.
3.) My theme is the Seamonkey 2.53.16 Default theme.

Thanks!

Steve

[n0spam]

View -> Show/Hide -> Website Navigation Bar.

[vtmozilla]

Thank you n0spam!