From MozillaZine Knowledge Base
Some web sites choose to open certain links in new windows. However, many users prefer using tabs to multiple browser windows. This preference allows for fine-tuning of browser.link.open_newwindow.
Possible values and their effects
Divert all links according to browser.link.open_newwindow. (Default in Firefox 1.0.x and SeaMonkey 1.0 to 2.1a2)
Do not divert any links (browser.link.open_newwindow will have no effect).
Divert all links according to browser.link.open_newwindow, unless the new window specifies how it should be displayed. (Default in Firefox 1.5 and later, in SeaMonkey 2.1a3 and later, and Camino 1.5)
This does not include:
First checked in
Has an effect in