I was testing some WordPress plugin playing with rss feed, and I found no matter how I change the settings (both WordPress and the plugin), the output of the feed will always be excerpt(in Firefox, not RSS Reader).
The thing is Firefox treat feed from WordPress a little bit different from other browsers, ie, with the same feed address, I got full content in IE.
After a search, I found the solution and technique explanation, see how to force Firefox to display your full feeds.
You needs some knowladge about WordPress and coding to understand, but it’s not necessary to learn why it’s happening, you can just follow the steps in the article and have it done.
I suppose most users subscribe feed using Clients such as Google Reader, but not the browser itself. The trick may be more valuable for bloggers who want to control there feeds perfectly.