In Firefox version 2 browers, links to new windows open in new browser tabs by default.  This means, when using My Stuff, your Stuff will appear in a new tab instead of a new window.  When you add an item to Stuff you will need to visit the tab to see it; the tab will not jump to the front.

It is possible to switch off this feature in the browser options.  Choose Tools, Options, Tabbed Browsing to find the controls.