It is easier to adjust the parameters of a sketch if you put them in constants:

1. import processing.opengl.*;
2.  
3. int DROP_NB = 10;
4. int RAIN_Z = 600;
5. int[] rainX = new int[DROP_NB];
6. int[] rainY = new int[DROP_NB];
7. int rainZ = 600;
8.  
9. void setup() {
10.   size(600, 400, OPENGL);
11.   noStroke();
12.   noCursor();
13.   frameRate(10);
14.   camera(width/2, 400, 560,
15.       width/2, height/2, 200,
16.       0, 1, 0);
17. }
18.  
19. void draw() {
20.   background(255);
21.  
22.   fill(0);
23.   rect(0, 0, width, height);
24.  
25.   for (int i = DROP_NB-1; i > 0; i--) {
26.     rainX[i] = rainX[i-1];
27.     rainY[i] = rainY[i-1];
28.   }
29.  
30.   rainX[0] = (int)random(mouseX-50, mouseX+50);
31.   rainY[0] = (int)random(mouseY-50, mouseY+50);
32.  
33.   sphereDetail(6);
34.   fill(70, 130, 180);
35.   for (int c = 0; c < DROP_NB; c++) {
36.     pushMatrix();
37.     translate(rainX[c], rainY[c], rainZ);
38.     sphere(6);
39.     popMatrix();
40.     rainZ -= RAIN_Z / DROP_NB;
41.   }
42.   rainZ = RAIN_Z;
43. }

And, please, try and find better subject line (how many "help me" topics you can find on this forum? too much!), like "adjusting rain fall simulation" or something.

I'm going to have this rain simulator running during a game. Is there a different option besides frameRate()?  Something involving adjusting some numbers that I'm not seeing?

Perhaps like this:

1. import processing.opengl.*;
2.  
3. int DROP_NB = 10;
4. int RAIN_Z = 600;
5. int RAIN_RATE = 5;
6. int[] rainX = new int[DROP_NB];
7. int[] rainY = new int[DROP_NB];
8. int rainZ = 600;
9.  
10. void setup() {
11.   size(600, 400, OPENGL);
12.   noStroke();
13.   noCursor();
14.   frameRate(10);
15.   camera(width/2, 400, 560,
16.       width/2, height/2, 200,
17.       0, 1, 0);
18. }
19.  
20. void draw() {
21.   background(255);
22.  
23.   fill(0);
24.   rect(0, 0, width, height);
25.  
26.   sphereDetail(6);
27.   fill(70, 130, 180);
28.   for (int c = 0; c < DROP_NB; c++) {
29.     pushMatrix();
30.     translate(rainX[c], rainY[c], rainZ);
31.     sphere(6);
32.     popMatrix();
33.     rainZ -= RAIN_RATE;
34.   }
35.  
36.   if (rainZ < RAIN_RATE)
37.   {
38.     rainZ = RAIN_Z;
39.  
40.     for (int i = DROP_NB-1; i > 0; i--) {
41.       rainX[i] = rainX[i-1];
42.       rainY[i] = rainY[i-1];
43.     }
44.    
45.     rainX[0] = (int)random(mouseX-50, mouseX+50);
46.     rainY[0] = (int)random(mouseY-50, mouseY+50);
47.   }
48. }

But it isn't very realistic, all these drops at the same level...

do you know this:

http://forum.processing.org/topic/3d-weather