tabs.captureTab()

let capturing = browser.tabs.captureTab(
  tabId,               // optional integer
  options              // optional extensionTypes.ImageDetails
)

function onCaptured(imageUri) {
  console.log(imageUri);
}
function onError(error) {
  console.log(`Error: ${error}`);
}
browser.browserAction.onClicked.addListener(() => {
  let capturing = browser.tabs.captureTab();
  capturing.then(onCaptured, onError);
});