Steps to set up Mozilla Thunderbird for ProtonMail Bridge :
1. In Windows and Linux, open Thunderbird, right-click in the sidebar → Settings.
In macOS, go to Tools → Account Settings.
2. Click Account Actions and select Add Mail Account from the dropdown menu.
3. Enter your name, email address, and ProtonMail Bridge password from your ProtonMail Bridge account settings.
The rest of the settings (security settings, ports, etc.) should be filled in automatically, but you can check this by clicking on the Manual Config button.
You also need to click Manual Config if you are using a custom domain. Enter all the settings provided in ProtonMail Bridge by clicking on Mailbox configuration under your account username. Do not use your regular ProtonMail account details.
4. You will be asked to confirm a security exception for Port 1143 on server hostname 127.0.0.1. This is your localhost port, which means that the certificate we provide must be self-signed, and is therefore not trusted by default in Thunderbird.
5. Thunderbird will now fetch your emails, which can take up to five minutes. You can send emails immediately.
You will need to configure some additional settings for Thunderbird to work correctly with your ProtonMail account.
6. In Windows and Linux, go to your Inbox, right-click your account name → Settings. In macOS, go to the menu bar → Tools → Account Settings…
7. Go to Copies & Folders and disable Place a copy in. Then scroll down to Drafts and Templates and make sure that drafts are kept on Local Folders. (On some Thunderbird setups, Local Folders is instead called your default account name).
Note: Drafts are saved on your local device only. If you create a draft in Thunderbird on your laptop, it will only be saved and accessible on that device.
You can now use Thunderbird with ProtonMail Bridge.