Firefox shows the Navigation Toolbar for me on Linux when tabs are not on top (i.e. browser.tabs.onTop pref = false).
Current Firefox versions only show the "Tabs on Top" menu entry in the "View > Toolbars" and "Firefox > Options" menu and in toolbar context menus if the tabs aren't in the default position on top.

Note that this pref will no longer have effect once the Australis code lands on the release channel (the Australis code is currently only on the Nightly update channel in the current 29.0a1 version).

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#nav-bar:not([collapsed="true"]){ visibility: visible !important; }

Indeed you're right. That works on Windows as well. Thanks, it can be a work around for now, but if my mind is not playing tricks with me, I distinctly remember making it happen even with tabs on top and then reverting to the default no-nav-bar scenario just a few weeks back.

My Googling skills are deceiving me for now, but let's see if somebody has any other other ideas before marking this as solved.

Cheers :-)

EDIT: Installing the "Always Awesome Bar" add-on is an way of doing it as well, but I was hoping for an about:config tweak (change or addition).