Can on open WiFi be secure?
A Freifunk router supplies an open WiFi without encryption.
This video shows you why this is not a problem (in German).
Alexander Lehmann, Lena Schall und Florian Maerlender
Link to the recommended plugin: HTTPS Everywhere
The video was created in 2015. Nowadays, most websites automatically forward to HTTPS and therefore encrypt the connection between your mobile phone/laptop and the sever that hosts the website. It is absolutely irrelevant if your computer uses WiFi with a totally complex password or Freifunk.