[jQuery] How to select an element based on visibility

[zephyr]

Hi,I cannot get my finger behind this one:

I have this HTML:
<p class="firstLine">This is the first line of the first paragraph</p>
<p>And here is some more text. Bladibladibla</p>
<p class="firstLine">This is the first line of the secons paragraph</
p>
<p>And here is some more text. Bladibladibla</p>
<p class="firstLine">This is the first line of the third paragraph</p>
<p>And here is some more text. Bladibladibla</p>

when I click a <p> class firstLine, I want the paragraph immediately
_below_ it to disappear if it is visible. If it is invisible, I want
it to show again.

This
$(document).ready(function(){
        $(".firstLine").click(function(){
                var el = $(this).next();
                $("el:hidden").click.slideDown("slow");
                $("el:visible").click.slideup("slow");
        });
});

doesn't work.
How can I do this?

Thanks