Can't move FF Bookmarks

I recently installed FF (1.5.0.4) for the first time on a new PC (Windows
XP Pro). I exported the bookmarks from another PC and imported them here.

When I go into Manage Bookmarks, I can drag and drop a bookmark from one
folder to another, but if I want to change a bookmark's position within a
folder, nothing happens. I can drag it to the new location, but when I
release the mouse button, nothing happens. All bookmarks remain in
alphabetical order.

Works OK on the original PC.

Is there some switch I need to flip to get thsi to work?

Thanks

Porter Smith