It's been in Google Chrome for some time, but it's only recently the SPDY (pronounced "Speedy") protocol has blazed a path onto another browser — in this case Firefox. Come version 11, Mozilla's popular open-source internet window will be able to communicate with SPDY-compatible servers, delivering improved browsing performance compared to the ancient, but functional, HTTP.
Google's keen to see widespread adoption of SPDY, going so far as to snag a trademark for it. The protocol doesn't use a single trick to boost speeds, it's a combination of techniques including header compression, multiplexing (combining data streams over a single connection), request prioritisation and server / client hints for content. Google's aiming for a 50 per reduction in page load times, and according to its SPDY whitepaper, is close to achieving this figure.
The only problem is, SPDY requires a secure connection facilitated by SSL to work, as an important part of the protocol uses TLS (Transport Layer Security). While not absolutely necessary, that's how it's being implemented (to promote a more robust and secure internet) and it doesn't look like the situation will change any time soon. This is why Google's only made it available on its secure services such as Gmail. There's an Apache 2 module available, but it's currently a beta and not recommended for production purposes.
Long story short, SPDY doesn't magically make the internet faster, servers must use SSL and TLS before anything can happen.
On a related note, Firefox 11 will support data migration from Chrome, if you miss the companionship of the little orange guy and his world-hugging proclivities. Going by the current release schedule, the updated browser will be available from March 13.