The behavior of opening a link from an external application is controlled via this pref on the about:config page. To open an external link you need to set this pref to '1'. The default is '-1' and this means the same action as browser.link.open_newwindow.

• browser.link.open_newwindow.override.external = 1

See this article from the values of this pref. See:

• http://kb.mozillazine.org/browser.link.open_newwindow
  1: current tab; 2:new window; 3:new tab;

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

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

Hmm, this is not exactly what I asked for. I would like '1' *but* on condition that this is 'blank' page. Setting it to '1' will always replace current tab, whether I have some page there or not.

Modifying this pref is the only way to get the link opening in the current tab and not in a new tab or new window. There is no way to make this only work in a blank tab. You can consider to always use a new window or new tab for external links if you do not want lose the current tab. You can always drag a tab from one window to the tab bar (plus button) in another window.

Relevant issue in bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=320989 (It's only 13y old)