When I was looking at this I searched for an about:config setting, but as best I can tell there is no pre-entered setting for this. I'm sure there is a string you can enter to control it manually. There shouldn't be any need to, though, if they're using a recent branch off the Firefox source tree.
quote:SPDY is similar to HTTP, with particular goals to reduce web page load latency and improve web security.
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An independent study shows that, in testing, the page load time with SPDY is not significantly different on most websites from HTTP or HTTPS, because old optimization techniques such as splitting the content between many hosts prevent pipelining from taking place.