If I have this:

Code HTML4Strict:
<div class="container">
<div class="first inner">ksjdkaskdksd content</div>
<div class="middle inner">just a line of content at the bottom</div>
<div class="last inner">ksjdkaskdksd content</div>

And want the middle div to have the same height as its first siblings (class "first"). How could I arhieve this using Javascript?

EDIT: the height of the divs are dynamic, as it is a responsive site.