UPDATE 2.5.2009: The author has updated the plugin to version 1.4.1 which fixes this problem.
If you use wp-twitter 1.4 then your rss feed does not work anymore because it is printing 2 returns which the rss feed of Firefox does not like. To fix this open wp-twitter.php and look for
(line 155-157) just above wp_twitter_add()
Put them together to ?><?php or remove them. save and your rss feed should work fine in firefox too.
Here is a link to the fixed version:
It seems this problem does happen on different version as well. Look for extra returns that should not be there.