It would be much better still for them to implement WebSocket: this is a marginal optimization at best, while adopting WebSocket would be a fundamental improvement and would much more significantly reduce bandwidth, etc.
I'm not sure it's marginal though. Consider a site that has a gallery of 100 thumbnails, each one say 200 bytes in size.
With firefox3, the browser would have to send 49,500bytes of HTTP headers just to get those 100 thumbnails which are only 20,000bytes in size. That's a ridiculous overhead. Even if you're using deflate or gzip, that's only gzipping the 20k data, not the 49.5k of headers.
http://www.w3.org/html/wg/html5/#network