SPDY indicator for Firefox 11+
Unfortunately there is no clue to when SPDY is actually working in Firefox because the only way to know is to examine the response headers for X-Firefox-Spdy.
SPDY is not on by default in Firefox 11, you have to go into about:config and change network.http.spdy.enabled to true.
Then watch for green lightning bolt the extension adds to the url address bar by visiting most google services or now Twitter.
Here are some SPDY enabled sites to test it on
(if you are aware of other SPDY enabled sites, let me know?)