Yes latest Firefox is meant to run on later kernels of Ubuntu but daper has an earlier kernel. I have tried this on one of my laptops for backward compatibility issue I was forced to install Ubuntu 6.06. So far this version of ubuntu is indeed very stable and fast. No regrets.
The audio should have run automatically. Choose alsa driver and please check the "pcm" volume and muting buttons; also check the mutings on the equalizer; mostly in my installations this is only a matter of switching: this one has seem defaulted on mute for manual switching to protect the speakers. I assure you by my experience this version of ubuntu is good and has given me no issues on audio.
If still audio doesn't come up check your external volume control and wire connections.
And still if issue persists do uninstall the alsa driver, reboot, then install it again; don't play an mp3 yet, look for a wave or ogg file to test play the new setup. check the pcm volumes and mutings. Do not go upgrades yet until you are convinced that audio is running locally.
Some audio files have codecs copyright protected: mp3 is one. So read a little about license issues then you may proceed to install additional codecs.
Hope this helps.