Sadly it looks like supporting SPDY for Firefox has to be special-cased (the indicator checks for X-Firefox-Spdy: in the headers). Because of this only a couple of the above servers support it.
@Steve, firefox itself injects that header, the server doesn't send it.
Unfortunately firefox doesn't seem to currently set any kind of accessible variable to indicate SPDY so I suggested an idea to detect that injected header for a Firefox port of the chrome indicator extension. Cheng Sun made it an hour later.
Because right now you have to intentionally "hack" your server to include it, once it's part of some standard components, it will be used automatically, or at least will be easy enough to add. I think nginx is working on including it in the default setup.
To sum it up, clients (SPDY enabled by default in FF13 and already in Chrome) and servers (Apache, nginx, Jetty, node, Google sites) are becoming available, some niches (mobile, high latency) are bound to benefit a lot and it's a good (better?) solution for the general case.