Fix term copy and paste in FF on windows

Author: Yang Zhen

app/styles/core-ui/Term.styl

@@ -241,3 +241,16 @@
     width: 13px;
   }
 }
+
+@-moz-document url-prefix() {
+  .xterm .xterm-helpers {
+    bottom: 0;
+    right: 0;
+    left: 0;
+  }
+  .xterm .xterm-helper-textarea {
+      left: 0;
+      width: 100%;
+      height: 100%;
+  }
+}