Customize Open/Save Dialog Sidebars
You can customize the sidebar in an Open/Save dialog the same way you can in a Finder window: you can reorder the position of items, drag them off into puff-of-smoke oblivion, and even drag a folder from the dialog's list into the sidebar.
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However, gray text in the interface usually indicates static text, which is not the case for URLs in the Firefox address bar. In the "Bug or Feature" contest this ranks as a fug (kind of a feature, but mostly a bug). Also known as a UAHSE (usability analyst's head-space error).