selectTab extension

background.js

browser.commands.onCommand.addListener(async (command) => {
  let num = parseInt(command.substr(9, 10)) - 1;
  let tabs = await browser.tabs.query({currentWindow: true});
  if (tabs.length < num) {
    return;
  }
  browser.tabs.update(tabs[num].id, {active: true});
});

manifest.json

{
  "applications": {
    "gecko": {
      "id": "selecttab@braniecki.net",
      "strict_min_version": "48.0"
    }
  },
  "manifest_version": 2,
  "name": "SelectTab Gnome Shortcut Override",
  "version": "1.0",

  "description": "An extension that overrides the default select-tab modifier key.",

  "permissions": ["tabs"],
  "background": {
    "scripts": ["background.js"]
  },
  "commands": {
    "selectTab1": {
      "suggested_key": { "default": "Ctrl+1" },
      "description": "Activate Tab 1"
    },
    "selectTab2": {
      "suggested_key": { "default": "Ctrl+2" },
      "description": "Activate Tab 2"
    },
    "selectTab3": {
      "suggested_key": { "default": "Ctrl+3" },
      "description": "Activate Tab 3"
    },
    "selectTab4": {
      "suggested_key": { "default": "Ctrl+4" },
      "description": "Activate Tab 4"
    },
    "selectTab5": {
      "suggested_key": { "default": "Ctrl+5" },
      "description": "Activate Tab 5"
    },
    "selectTab6": {
      "suggested_key": { "default": "Ctrl+6" },
      "description": "Activate Tab 6"
    },
    "selectTab7": {
      "suggested_key": { "default": "Ctrl+7" },
      "description": "Activate Tab 7"
    },
    "selectTab8": {
      "suggested_key": { "default": "Ctrl+8" },
      "description": "Activate Tab 8"
    },
    "selectTab9": {
      "suggested_key": { "default": "Ctrl+9" },
      "description": "Activate Tab 9"
    }
  }
}

Ctrl+9 should activate the last tab instead of the 9th tab to match the behaviour of Alt+9.