custom session variables being lost in FF [closed]

Are you sure it works in other browsers?

WordPress by default drops/unset all unknown variables. Check wp-includes/load.php inside the function wp_unregister_GLOBALS().

I had similar session variable issue and fixed it using (in functions.php)

<?php
// Enable session
add_action('init', 'custom_init_session', 1);
function custom_init_session() {
    if (!session_id())
        session_start();
}

ie. Call session_start() during init instead of in header.php.

Give it a try!