I see this on its extension's page

• https://addons.mozilla.org/firefox/addon/single-key-tab-switch/

To configure key bindings, enter about:config in your address bar, and search for the following properties: extensions.skts.prevTabKey extensions.skts.nextTabKey These properties should be set to ONE character only - the character for the key that should trigger the tab switch. Default settings: extensions.skts.prevTabKey=1 extensions.skts.nextTabKey=2

So you need to locate and modify these two prefs.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

The SKTS extension only supports a single key for these actions, so you can't use virtual keys like the cursor keys.

• chrome://skts/content/skts.js
  if (key === skts.prevTabKey.charCodeAt(0)){}
  else if (key === skts.nextTabKey.charCodeAt(0)){}

Sorry, I forgot to ask - how to specify those keys in computer terms - is what I meant. I didn't understand why can't I use them; these are single keys; and what does "virtual" mean?

You can only use un-shifted key from the keyboard.

See this about virtual key codes:

• https://developer.mozilla.org/Web/API/KeyboardEvent/key

(edit: fixed link)

You could use 4 and 6 on the numeric key pad with num lock enabled.

I was going to use them without shift or any other moderating key. I don't have the num keys. :-)