How do Jehovah Witnesses support scripturely the idea that humans will live on earth after death? A Jehovah Witness and I spoke the other day and he explained to me that JWs believe that we'll live on earth after we die.

What scripture, if any, do they use to back this belief?
The idea comes from Revelation 21
Revelation 21:1 (NIV)

1 Then I saw “a new heaven and a new earth,” for the first heaven and the first earth had passed away, and there was no longer any sea.

The problem with interpreting Revelation is that there's a lot of debate on what it actually means.
Will we live on a new earth?  Yes in the sense that God will give us a place to live.
Will it be identical to the old earth (with tornadoes, volcanoes, etc)?  Who knows!
The New World Translation of this doesn't really vary.  You can read that if you'd like.