Opening firefox pages in the background

This from...
http://www.techsupportalert.com/content/how-stop-firefox-stealing-window-focus.htm

First, type (or copy and paste) "about:config" (minus quotes) into the
Firefox address bar and hit enter.
The next thing you'll see is a window telling you this might void your
warranty (a little humor there, Firefox doesn't have a warranty).

Click the "I'll be careful, I Promise!" button. The about:config page will open.
In the Filter bar, type (or copy and paste)
"browser.tabs.loadDivertedInBackground" minus the quotes. You will

then see an entry under Preference Name that matches what you typed
in.

By default, the value of this entry is false. To change it, double
click the entry and it will toggle so that the value reads true.
Restart Firefox and you're done.

If you need to undo the change, right click on the entry, choose Reset
and restart Firefox. This changes the entry back to its default
setting. You can reset any entry that's been changed in the
about:config page the same way. If you scroll down, you'll see some
entries in bold. Those are entries that have been changed already,
either by setting preferences (in the Tools/Options menu) or by an
add-on. Right clicking any entry in bold and choosing Reset will cause
it to revert to the default entry.

Changing entries in the about:config page can cause Firefox to behave
oddly or not work at all. This isn't a good area to experiment with
unless you'd like to uninstall and then do a clean reinstall of
Firefox.