I'm surprised you were able to force 'h263' codec video into a FLV container. The problem is that Flash Player does not support H.263 video. It supports Sorenson Spark, which is a variation of H.263. Access it using the 'flv1' video codec instead.
As an aside, execute ffmpeg -codecs | grep 263
to get an idea for how many variations of H.263 exist.