This Firefox extension allow to automatically opt-out of Google cookies when using Incognito mode.
How it works: when a new private Firefox window is created, it checks if specific Google cookies are set. If they are not, it creates them with values that make it opt-out of Google tracking... I guess. It's a mess. But hey, at least, the prompt is no longer displayed.
Limitations: for now, works only for google.com, not other Google TLDs
