Swapping between two browser context menus depending on the amount of selected tabs.

background.js

let single = true

const menus = {
  single: browser.menus.create({
    contexts: ['tab'],
    title: '&Close Tab'
  }),
  selection: browser.menus.create({
    contexts: ['tab'],
    title: '&Close Tabs',
    visible: false
  })
}

browser.menus.onShown.addListener(async (info, tab) => {
  if (!info.contexts.includes('tab')) return

  if (
    (!tab.active && tab.highlighted === single) ||
    (tab.active && (await browser.tabs.query({
      windowId: tab.windowId, highlighted: true
    })).length === 1 !== single)
  ) {
    single = !single
    browser.menus.update(menus.single, { visible: single })
    browser.menus.update(menus.selection, { visible: !single })
    browser.menus.refresh()
  }
})