Set autospark off and try to spark the fire using your mouse.

Change the settings, click restart and see what happens.

The inner medium is a circle in the center of the canvas with the radius of 150px.
The outer medium is what's left from the drawing area.

The mask is a vector that defines what neighboring cells will be counted as alive.
Possible values: 0 - ignore the cell, 1 - take it into consideration.
For each cell, the neighboring cells will be identified like this:

0 1 2
7 x 3
6 5 4

So a mask like this: 1, 1, 0, 1, 1, 1, 0, 1 will take into consideration only the neighbors 0,1,3,4,5 and 7.

The change rules control the next state of the cell depending on the number of neighboring cells that are alive.
The index represents the number of alive cells.
Possible values:

0 - mark the cell as dead
1 - mark the cell as alive
i - flip the state of the cell
k - keep the current state of the cell

If the rules vector is 0,0,1,0,i,0,0,k,0 then for:

0 neighbors alive - the cell is marked dead
1 neighbor alive - the cell is marked dead
2 neighbors alive - the cell is marked alive
3 neighbors alive - the cell is marked dead
4 neighbors alive - the state of the cell is flipped
5 neighbors alive - the cell is marked dead
6 neighbors alive - the cell is marked dead
7 neighbors alive - the state of the cell is not changed
8 neighbors alive - the cell is marked dead