SPDY isn't enabled by default until Firefox 13 (currently on the beta channel, scheduled for release in four weeks).

I wonder if that's what this is though: Ubuntu LTS having modified the FF build they run to use SPDY by default (given the LTS is supposed to be out for 5 years now, they may have chosen to jump the gun on that feature), and the SPDY support in the bundled FF isn't complete?