In some cases, your wordpress do not generate a valid rss; Weirdly, your rss includes an empty line at the top of the feed. In such a case, when viewed on Firefox3.5 or validated with Feed Validator, your rss would fail.

Solution:

What we have to do is just to remove an empty line, but to find the bug source is not easy. The output feed is generated through mulitple processes inside Wordpress, themes, and plugins.

The point is that

1. Check your wp-config.php that there are no trailing empty lines before <?php and after ?>
2. Check functions.php in your theme, too.
3. Check other theme php files.
4. If you still cannot solve the problem, stop each plugin and re-generate RSS. Sometimes an empty line is included in plugin php files.

Hope your luck.