Opera also has built-in RSS, dunno about Atom. However, I think Firefox's RSS feature is far superior to Opera's, even though I am using Opera instead of Firefox.
Hard to explain the difference without screenshots, which I cannot be bothered with.

Firefox lists RSS feed headlines and automatically updates RSS feed listing and removes older RSS feeds when they are removed the the originating site.
Opera lists RSS feeds in e-mail like manner and does not delete old RSS feeds until user deletes them, very e-mail like manner.
Some users might prefer Opera's RSS feed because Opera also shows more than just headlines, with Opera users need to visit the originating site even less frequently than with Firefox, as Firefox shows only the headline. I prefer Firefox's style though, it is simple but yet effective way. I wish Opera could be used in same way, maybe it becomes possible in later versions.
In any case IceFire, just for the heck of it, go to two of your favorite news sites for example, get their RSS feed. Now, use it a day or two. It's very convenient way to stay on top of things without actually visiting the site until you see something interesting.