diff --git a/src/js/tools/drawing/DitheringTool.js b/src/js/tools/drawing/DitheringTool.js
index 7477fdf9..505cf86b 100644
--- a/src/js/tools/drawing/DitheringTool.js
+++ b/src/js/tools/drawing/DitheringTool.js
@@ -17,12 +17,17 @@
    * @override
    */
   ns.DitheringTool.prototype.applyToolAt = function(col, row, color, frame, overlay, event) {
+    // On Firefox/IE, the clicked button type is not part of the mousemove event.
+    // Ensure we record the pressed button on the initial mousedown only.
+    if (event.type == 'mousedown') {
+      this.invertColors_ = event.button === Constants.RIGHT_BUTTON;
+    }
+
     // Use primary selected color on cell with either an odd col or row.
     // Use secondary color otherwise.
     // When using the right mouse button, invert the above behavior to allow quick corrections.
     var usePrimaryColor = (col + row) % 2;
-    var invertColors = event.button === Constants.RIGHT_BUTTON;
-    usePrimaryColor = invertColors ? !usePrimaryColor : usePrimaryColor;
+    usePrimaryColor = this.invertColors_ ? !usePrimaryColor : usePrimaryColor;
 
     var selectedColors = pskl.app.selectedColorsService.getColors();
     var ditheringColor = usePrimaryColor ? selectedColors[0] : selectedColors[1];