Is SPDY any different than http multiplexing over keep alive connections

Question

HTTP 1.1 supports keep alive connections, connections are not closed until \"Connection: close\" is sent.

So, if the browser, in this case firefox has network.http.pipel

Answer 1

SPDY has a number of advantages that go beyond what HTTP pipelining can offer, which are described in the SPDY whitepaper:

1. With pipelining, the server still has to return the responses one at a time in the order they were requested. This can be a problem if the client requests a resource that's dynamically generated before one that is static: the server cannot send any of the "easy" static responses until the dynamically generated one has been generated and sent. With SPDY, responses can be returned out of order or in parallel as they are generated, lowering the total time to receive all resources.
2. As you noted in your question, not all servers are able to deal with pipelining: it's not just load, some servers actually behave incorrectly when the client requests pipelining. Using a header to indicate that it's okay to do pipelining is too late to get the maximum benefit: you are already receiving the first response at that point, so while you can use it on future connections it's already too late for this one.
3. SPDY compresses headers using an algorithm which is specific to that task (stateful and with knowledge of what is normally in HTTP headers); while yes, SSL already includes compression, just compressing them with deflate is not as efficient. Most HTTP requests have no bodies and only a short GET line, so the headers make up virtually the entire request: any compression you can get is an improvement. Many responses are also small compared to their headers.
4. SPDY allows servers to send back additional responses without the client asking for them. For example, a server might start sending back the CSS for a page along with the original HTML, before the client has had a chance to receive and parse the HTML to determine the stylesheet URL. This can speed up page loads even further by eliminating the need for the client to actually parse the HTML before requesting other resources needed to render the page. It also supports a less bandwidth-heavy version of this feature where it can "hint" about which resources might be needed, and allow the client to decide: this allows, for example, clients that don't care about images to not bother to request them, but clients that want to display images can still request the images using the given URLs without needing to wait for the HTML.
5. Other things too: see William Chan's answer for even more.