Firefox drops the session variable

I just have a simple session start and assigned a session variable on login. However when I switch pages, the session variable does not stick in Firefox. I can still see the session is there but just not the variable. In IE7, IE8, Chrome, and Safari, it works and sticks. What is happening in Firefox and how can I fix it?

Here is a snippet of the code.

session_start();

if ($_POST['login']) :
$_SESSION['loggedin'] = 1;
endif;