Probably navigator.webdriver, but there are multiple properties.

https://antoinevastel.com/bot%20detection/2017/08/05/detect-...

https://antoinevastel.com/bot%20detection/2018/01/17/detect-...

That's why I was wondering. Last time I checked headless Chrome could be pretty reliably detected in a number of ways, as you say. That they mention just one variable seems quite odd, given that they position themselves as specialists in the field.

The webdriver property is as far as we know the only one that stays different if you use non-headless chrome with puppeteer. Rest can be handled by use of non-headless chrome as mentioned in the article.

But you are right, after reading through it again, this section of the article should be improved.

Is there anything like this to reliably detect Firefox headless?