Firefox dragging fix

src/tool-selection.js

@@ -76,9 +76,13 @@ function setSelectedTool(newSelectedTool) {
   toolElements[newSelectedTool].classList.add("selected")
 }
 
+function withPx(str) {
+  return `${str}px`
+}
+
 function updateCanvasOffset() {
-  HTML.canvas.style.left = canvasOffset[0]
-  HTML.canvas.style.top = canvasOffset[1]
+  HTML.canvas.style.left = withPx(canvasOffset[0])
+  HTML.canvas.style.top = withPx(canvasOffset[1])
 }
 
 function centerCanvas() {