How to open an URL from command line to Firefox and "switch to tab"?

You can do it like:

firefox -new-tab "www.duckduckgo.com"

Well at least one possible method is to modify your about:config files. See http://www.ghacks.net/2009/07/03/force-firefox-to-open-links-in-same-tab/ for details as to how to do this.