It looks like this might be related to a bug in jQuery.
https://stackoverflow.com/questions/8389841/using-jquery-to-determine-selected-option-causes-specified-attribute-is-depreca
https://bugs.jquery.com/ticket/12072
However, according to these posts it should only impact users of Firefox 7. What version of Firefox are you using?
It is possible you are actually encountering two errors but only seeing a message from the one because it is Firefox specific.
I will look into this more and see if I can reproduce it.
