If you replace the "window" with "tab" so it reads ",new-tab" you won't destroy the current tab, but add a new tab. I likes it!
But that way you'll override whatever preference you set in Firefox through the Preferences dialog (just as today "new-window" does). "/usr/bin/firefox" is not a good place to enforce preferences (and that's why the current behaviour is considered a bug).
-- Mariano
