I have looked at a lot of Firefox extensions over time. One of the most useful that I have come across of late is Tab Mix Plus.  This extension gives the user vastly more control over tabbed browsing than un-extended Firefox. 

Whilst I have had the standard Firefox settings configured to open all new windows in new tabs, my online banking (ANZ) has persistently opened its logged-in session in a separate Window.  Due to some problems that I have been experiencing with the Fluxbox window manager and xinerama, the banking window inevitably opened very small on my smallest screen.  Not any more!  Now when I log into banking, I get a nice, new tab in my existing window.  That single feature makes Tab Mix Plus worthwhile for me.