Go to a window with any editable text field (including URL bar- any page, composer doc, new message) Type "foo bar" Select foo. Copy. select bar. put cursor in edit field, and click middle button. Note that it pastes the clipboard (foo), when it should copy the selection (bar) and paste it. It should only paste the current clipboard if there is no selection.
*** This bug has been marked as a duplicate of 7787 ***
[bugday] verifying duplicate. middle click auto copying bug.