use key-code

src/background/index.js

@@ -1,16 +1,16 @@
 import * as tabs from './tabs';
 
 const KEY_MAP = {
-  'tabs.prev': 104,
-  'tabs.next': 108,
-  'scroll.up': 107,
-  'scroll.down': 106
+  'tabs.prev': KeyboardEvent.DOM_VK_H,
+  'tabs.next': KeyboardEvent.DOM_VK_L,
+  'scroll.up': KeyboardEvent.DOM_VK_K,
+  'scroll.down': KeyboardEvent.DOM_VK_J
 };
 
 browser.runtime.onMessage.addListener((request, sender, sendResponse) => {
   let response = null;
 
-  switch (request.key) {
+  switch (request.code) {
   case KEY_MAP['tabs.prev']:
     tabs.selectPrevTab(sender.tab.index);
     break;

src/content/index.js

@@ -11,9 +11,9 @@ const invokeEvent = (type) => {
   }
 }
 
-window.addEventListener("keypress", (e) => {
+window.addEventListener("keydown", (e) => {
   browser.runtime.sendMessage({
-    key: e.which || e.keyCode,
+    code: e.keyCode,
     shift: e.shift,
     alt: e.alt,
     meta: e.meta,