Tor is free software for enabling anonymous communication.
ProtonMail now has an official onion site:
To use our Tor hidden service (also known as an onion site), you must have Tor installed or use Tor browser. Instructions on how to set up Tor to access your encrypted mailbox can be found here.
Using Tor to access ProtonMail is optional, but in some situations, it may bring additional security benefits. More information about how ProtonMail utilizes Tor to protect your privacy can be found here.