This often (but not always) happens when I drag a XUL element, such as a tab or bookmark, or while customising the toolbar. If I ignore the assertion, the drag appears to proceed normally, with the intended drag image of that XUL element. It does not appear to happen if I drag a tab when I only have one tab open.
This assertion for me 100% of the time if I drag a history-entry or a bookmark from the History or Bookmarks menus. Also fires if I open Bookmarks | Show All Bookmarks and drag anything in the right pane around.
CC'ing roc, since he touched this assertion's line of code last, as part of bug 641426: https://hg.mozilla.org/mozilla-central/rev/90e46b3e8e6f#l19.12
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