Right click a blank space on a toolbar, and you will get the customize option. You will get a pane with icons that may be dragged on to the toolbar. The list tabs icon is the one I think you have dragged off. Alternatively use the reset option [restore default set]

I think it might be possible. Firefox allows you to override certain toolbar behaviors by creating a style sheet file named userChrome.css (or you can use the Stylish add-on). If I get a chance to test this later today I will post a specific suggestion.

The rule you would need to override is this one:

#toolbar-menubar:not([autohide="true"]) ~ #nav-bar > #bookmarks-menu-button-container,
#toolbar-menubar:not([autohide="true"]) ~ toolbar > #personal-bookmarks > #bookmarks-menu-button,
#toolbar-menubar:not([autohide="true"]) > #personal-bookmarks > #bookmarks-menu-button {
 display: none;
}

https://support.mozilla.org/media/uploads/images/2012-05-15-21-57-45-240bc0.png

You can move the Bookmarks Menu button with the drop marker to another toolbar (i.e. not the Navigation Toolbar) to have it visible when the menu bar is enabled.
I place that button on the left hand side of the tab bar.
On Linux, the Firefox button is located on the Tab bar, so I have both buttons next to each other if the menu bar is hidden.

Thanks, John99. Your response helped me to see the bookmarks icon where it had landed in the Navigation toolbar and I was able to drag it back into position. Thanks to all for your great suggestions.

Thanks for posting back, glad we helped with your problem.

I probably misunderstood the question slightly.

As this thread is about bookmarks it may also be worth noting that Firefox will display bookmarks in the bookmarks library/manager and in a sidebar. Both of these are accessible by using keyboard shortcuts,which may vary slightly on different OS s. In Windows the sidebar is Crl B and the Library is Ctrl Shift B. In Linux the library is Ctrl Shift O.

• see keyboard shortcuts_bookmarks