The ERROR:
If you use more than menu ITEMS to link FB and if there is one item which is set only for registered. And if you are not logged in and use this link index2.php?option=com_fireboard&func=fb_rss&no_html=1 will cause the message:
You are not authorised to view this resource.
You need to login.

The solution is, give above link with Itemid.
modify: com_fireboard/fireboard.php
at line 867
[code:1]
/index2.php?option=com_fireboard&func=fb_rss&no_html=1" target="_blank"

change it into

/index2.php?option=com_fireboard' . '&Itemid=' . FB_FB_ITEMID . '&func=fb_rss&no_html=1" target="_blank" [/code:1]

Hope this help for you.