Malís birthplace; his mom owned a large ranch here.
RPG Reference (from Serenity Roleplaying Game, ISBN: 1931567506)
Shadow was a prairie planet that took well to terraforming. It was known for its farms and ranches. The planet was almost entirely rural: no cities and only a few towns dotted the surface. The people of Shadow were hard workers and independent-minded. It was one of the first to stand against the Alliance. Most of the planet's young people volunteered to fight for Independence.
Shadow was aggressively bombed during the war in an attempt to break the Browncoats & teach them a lesson; it had the opposite effect.
Today, Shadow is a ghost planet. No one lives there. No one can.
[non-canon] Edit to non-canon, the entry in the RPG indicates that Shadow was bombed during the war - and was possibly one of the first targets - in an attempt to avoid ground battles. I haven't read the non-canon history, so I'm not sure if this would require a major change. As this was Mal's homeworld, it had a lot to do with him, as it was effectively destroyed. [/non-canon]
In River's classroom scene in the movie "Serenity", Shadow is the fourth planet listed on the screen to the left of the teacher.