Yes, Firefox has SPDY enabled by default starting with 13.0: http://www.mozilla.org/en-US/firefox/13.0/releasenotes/

Not just Google's servers, but any server where SPDY is available (e.g. Twitter).

This is handy: https://addons.mozilla.org/en-us/firefox/addon/spdy-indicato...

here's a counterpart for Chrome if anyone's interested: https://chrome.google.com/webstore/detail/mpbpobfflnpcgagjij...

I just installed it, seems to work as expected. Shows the indicator on both Google and Twitter

really though, just google's servers because google is the only spdy-enabled site where https is forced. twitter doesn't come in over spdy unless you type in https.

Twitter does have an "Always use HTTPS" option for when you're logged in.