We do not recommend synchronizing any email client with Protonmail as it will break the inbuilt security. Having said that Protonmail has a feature for you to do this called ProtonMail Bridge.
ProtonMail Bridge, allows you to use another client to connect to your Protonmail account (Not recommended).
- For Apple email and any other client: https://protonmail.com/bridge/
You can find the details here for Thunderbird: https://protonmail.com/bridge/thunderbird