This loads "View Page Source" and "View Selection Source" in tabs instead of new windows.
Full of features in "View Page Source" window is also available in tabs, and those tabs can be reopened by "Undo Close Tab" because this uses session store API.
By the way, this includes an UI to choose external viewer applications for Firefox 2's feature.
Middle-click or Ctrl-click (Command-click on Mac OS X) on "View Source" menuitems reverses the behavior. When you choose "Show source in window", middle-click shows page source in a tab. Otherwise, middle-click shows source in a window (like as Firefox's default behavior).
This is developed under a project to restructure TBE ( http://piro.sakura.ne.jp/xul/tabextensions/index.html.en
) for Firefox 2.