i'm using Firefox v1.5.0.4, and I don't have that problem with 100% cpu utilization when viewing RSS feeds in Firefox. Sometimes buggy extensions (or extensions being incompatible with each other) can cause that symptom at weird times, like maybe when viewing RSS feeds. So, check that your extensions are all updated (Tools >> Extensions, and click the Update Now button if available for any).
If you're a fan of RSS feeds, and find you read them a lot, I really like
FeedDemon for reading and tracking RSS feeds. Obviously it's not free like FireFox, but it's great value, and worth the money. They have a trial version you can check out on there.