tabTracker snippet

api.js

const {Management} = Cu.import("resource://gre/modules/Extension.jsm", {});

....

class API extends ExtensionAPI {
  getAPI(context) {
    const {tabTracker} = Management.global;
  
    return {
      mynamespace: {
        mymethod(tabId) {
          let tab = tabTracker.getTab(tabId);
          ...
        },
      },
    };
  }
}