SPDY has been enabled by default in Chrome for a while.
SPDY is enabled by default in Firefox 13, currently on the nightly channel. In Firefox 11 (beta channel) or 12 (aurora channel) [2] it can be enabled with a hidden preference [3].
After Firefox 13 is released in a few months, over 40% of web users worldwide will be browsing with SPDY. (Today the number is already over 25% worldwide thanks to Chrome alone, based on numbers from StatCounter.)