I found the answer over at Devils Workshop.
RSS 2.0 & ATOM have exclusive support for summaries. In RSS 2.0, actual content is placed in content element while summaries can be optionally put in description element. Similarly in ATOM, actual content is placed in content element while summaries can be optionally put in summary element.
Now other browsers displays data from content but Firefox uses data in description (for RSS 2.0) or summary (for Atom). These element are optional so by commenting them we force firefox to use data in content field. As both description & summary are optional element in their formats at item/post levels, feeds are still valid. In fact, blogger.com by default don’t use these elements so this problem does not arise there.
Actually this can be seen as a feature too as you can see entire feed without much scrolling. But firefox should somehow tell a user that full feeds area also available whenever applicable.