enhanced detecting '+' and '-' keys for Firefox

HISTORY.md

@@ -83,6 +83,7 @@
 ### 2019-05-21
 * Objects: fixed stage-size settings bug
 * new "World Map" library
+* enhanced detecting '+' and '-' keys for Firefox
 
 ### 2019-05-20
 * Objects, Blocks added plus- and minus- keys to key pressed predicate and hat block 

src/objects.js

@@ -7667,10 +7667,12 @@ StageMorph.prototype.processKeyEvent = function (event, action) {
     case 40:
         keyName = 'down arrow';
         break;
-    case 187:
+    case 171: // Firefox
+    case 187: // Chrome, Safari
         keyName = '+';
         break;
-    case 189:
+    case 173: // Firefox
+    case 189: // Chrome, Safari
         keyName = '-';
         break;
     default: