Thunderbird is quite separate from any and all email service providers. So you can set up Thunderbird to connect to whichever email server you use, or to multiple email accounts from different providers, for example, Yahoo, Outlook, a corporate business system, etc. You need to find out from ProtonMail what settings are needed for POP or IMAP and for SMTP, and the authentication settings (security/password requirements) in order for a mail client to connect to their service. Ask their support or find out online. You may even find that ProtonMail has a page of instructions specifically for Thunderbird, some email providers do. I highly recommend asking them - as I see lots of articles about ProtonMail online but nothing that I would trust is current, and they seem to have a separate Bridge software that you might need because of their encryption. When ready with that information to add an account in Thunderbird - here's how https://support.mozilla.org/en-US/kb/manual-account-configuration Note that this example uses Gmail settings - replace them with the correct ones for ProtonMail. You also need your email password (the same one you would use to log in to online webmail).

Some comments from people using Protonmail https://support.mozilla.org/en-US/questions/1357808 and a link to the proton setup instructions for Thunderbird in this answer https://support.mozilla.org/en-US/questions/1320034