pageAction.setPopup()

details

object.

tabId

integer. The ID of the tab for which you want to set the popup.

popup

string or null. URL to the HTML file to show in a popup.

If an empty string ("") is passed here, the popup is disabled, and the extension will receive pageAction.onClicked events.

If null is passed here, the popup is reset to the popup that was specified in the page_action manifest key.

Listen for tabs.onUpdated events, and switch the popup if the loading status changes:

browser.tabs.onUpdated.addListener((tabId, changeInfo, tabInfo) => {
  if (changeInfo.status) {
    browser.pageAction.show(tabId);
    if (changeInfo.status === "loading") {
      browser.pageAction.setPopup({
        tabId,
        popup: "/popup/loading.html"
      });
    } else {
      browser.pageAction.setPopup({
        tabId,
        popup: "/popup/complete.html"
      });
    }
  }
});