Sometimes the "Match case" button on the find bar gets checked, so make sure that isn't the problem. Some pages also have multiple frames and I've noticed Firefox sometimes doesn't search in all the frames. If there's a particular site where you see this, could you post the URL?

Thanks, that solved it!

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THANKS, jscher2000! This solution (MatchCase box checked) solved this problem for me. Don't know why I missed this solution for such a long time.