Transition guide for users of Classic

From the guide:

  (q) How do I permanently delete messages?
  (a) ... Use the % key (normally Shift+5) to delete the selected mail instantly, forever.

My question: How do I use the above "keyboard shortcut" on Firefox for Android to permanently delete (instead of moving messages to trash and then emptying the trash)?

I'll miss classic mode (and I don't care to use the FastMail app on Android when classic mode worked fine).