protonmail.xml

<?xml version="1.0"?>
<clientConfig version="1.1">
    <emailProvider id="protonmail.com">
      <domain>protonmail.com</domain>
      <domain>protonmail.ch</domain>
      <domain>pm.me</domain>

      <displayName>ProtonMail</displayName>
      <displayShortName>ProtonMail</displayShortName>

      <incomingServer type="imap">
        <hostname>localhost</hostname>
        <port>1143</port>
        <socketType>STARTTLS</socketType>
        <username>%EMAILADDRESS%</username>
      </incomingServer>

      <outgoingServer type="smtp">
         <hostname>localhost</hostname>
         <port>1025</port>
         <socketType>STARTTLS</socketType> <!-- see above -->
         <username>%EMAILADDRESS%</username>
      </outgoingServer>

      <enable
           visiturl="https://protonmail.com/bridge/">
           <instruction>You must first install and activate the ProtonMail Bridge.</instruction>
      </enable>

      <documentation url="https://protonmail.com/bridge/thunderbird">
         <descr lang="en">Configure Thunderbird for the ProtonMail Bridge.</descr>
      </documentation>

    </emailProvider>

    <webMail>
      <loginPage url="https://mail.protonmail.com/login" />
      <loginPageInfo url="https://mail.example.com/login/">
        <username>%EMAILADDRESS%</username>
        <usernameField id="username" name="username" />
        <passwordField id="password" name="password" />
        <loginButton id="login_btn"/>
      </loginPageInfo>
    </webMail>

</clientConfig>

This need to be put in: /usr/lib/thunderbird/isp/protonmail.xml