On Firefox 3.5, dragging and dropping selected web page text containing a single word to the Clippings status bar icon will cause the browser to start loading a URL containing that single word. Normally, when dragging and dropping any text to the status bar icon, the New Clipping dialog box should appear.
This issue isn't happening when dragging and dropping a selection containing multiple words.
To work around this, use the other options available for creating a new clipping. They are described in the Clippings Quick Start guide.
Thanks to the reporter of this bug for bringing this to my attention. Readers interested in the details can refer to bug 21223.