I've just had a quick look, also on FF3/Mac.
The issue is not that the select_list element is invisible - you can
address it correctly as show by calling:
Calling the "methods" method on the element reveals that the "set"
method you are calling is not present. I was surprised to see that
this method exists - I've always used the "select" method, which works
fine. The RDoc reveals that set should be an alias for select.
I would suggest using the select method for the time being:
However, if you wish to use the set method, add the following code to
your firewatir/htmlelements.rb file in the SelectList class beneath
the select method's definition:
alias :set :select