There is a problem (or feature) in Firefox. For wordpress blogs and few other feeds it always show partial content making it hard to judge for a wannabe subscriber whether your are displaying full feed or partial feed.
Many subscribers just hate partial feeds and most of them are unaware of this default firefox behavior.
As a blogger if you care about your subscriber count, you can use this hack to force firefox to display your original full feed.
Details are at:
[p.s. I don't know if external links allowed here but above is highly relevant.]