I eject the DVD, attempt starting Firefox, and put the DVD back in.
Firefox did not start, but even after inserting the live DVD disc back, Firefox was still returning an input/output error.
Apparantly, it is still considering the data as unreadable.
sudo sh -c 'echo 1 >>/proc/sys/vm/drop_caches' just made everything worse: not loading from DVD, just wiping the cache.
How do I force Linux to reload previously errornous data from the DVD?