Mod_rewrite problem

I have a strange problem with my new site here, at DH. It looks like mod_rewrite is not working. This is the entire contents of my .htaccess file, which resides at site root.

Options +FollowSymLinks

RewriteEngine On

RewriteBase /
RewriteRule ^/browsers/ /adxmenu/intro/ [NC,L]


ErrorDocument 404 /404.php

The first rule throws 404, even though /adxmenu/intro/ exists (I can type it in the address bar and get the page). However, 404 rule at the bottom does work.

This looks very basic - what am I missing here?