I surf in fullscreen-mode and never know if a new tab has opened, because i hide the tab-bar and firefox does not give any notification about new tab creation. Is there a lightweight way to get newtab notification (sound) ?
You could try the Noise firefox addon:
The Noise Firefox add-on can play sounds for more than 20 browser events such as when opening a new tab or when a popup has been blocked. You have the option to specify a different sound of your choosing for each action and you can also create your own sound events.