See:

• https://support.mozilla.org/kb/how-do-i-get-firefox-button
• https://support.mozilla.org/kb/Menu+bar+is+missing

Go to View > Toolbars and remove the check-mark next to the menu bar entry or right-click the menu bar or another toolbar.

But that makes the title bar look like this: http://i.imgur.com/0GZxW.png .

And of course, B would look like A whenever I press alt, ideally.

Do you have the Firefox window maximized?

If that is the case then does it help if you set the browser.tabs.drawInTitlebar pref to false on the about:config page?

• http://kb.mozillazine.org/about:config

Type about:config in the location (address) bar to open the about:config page and press the "Enter" key, just like you type the URL of a website to open a website.
If you see a warning then you can confirm that you want to access the about:config page.

• Use the Search bar at the top of the about:config page to locate preferences more easily
• Preferences that have been modified show as bold (user set)
• Preferences can be reset to the default via the right-click context menu if they are user set
• Preferences can be changed via the right-click context menu: Modify (String or Integer) or Toggle (Boolean) or by double-clicking the line with the pref
• Preferences can be created via the right-click context menu: New -> String, Integer, Boolean

See also:

• Personal Titlebar: https://addons.mozilla.org/firefox/addon/personal-titlebar/

Changing the about:config preference didn't make any difference. The extension is nice, maybe better than what I had in mind, however, still leaves me with the same problem, which is the custom titlebar (C in the image I posted earlier), while what I want is the regular windows titlebar (A and B).