Have you reviewed options at the protonmail site? It offers both a tool and also support to do that.

I have David and thank you. As is usual for me, it is far from clear. Happens every time I try to do something like this and I struggle. I knew this would not be easy that that's why I asked for a series of steps since the interoperability of these platforms is, in my opinion, not user friendly. Protonmail describes the process by going from link to link to link with "articles" which describe, in language that I find somewhat bewildering and, few of which seem to apply to me. I downloaded the Protonmail "import/export app" and that gives me two options to decide from where the folders I wish to import reside. I have chosen "from account" rather than "local files" which is hopefully correct. That leads to a box which requires four pieces of information and another link to another article regarding setting up IMAP for the incoming folders. The four pieces of information are 1) email, which I presume they mean an email address although they don't specify, a password, which I presume is for said email address, a server address, whatever that is, and a Port #, whatever that is. Also since the email provider that is linked to the TB account that has the local folders that I wish to export to Protonmail is Mailfence I'm not sure how the connection is made. Perhaps you or someone could help to clear up some of the mystery for me so that I can reduce the time wasted on this to a minimum.

Okay. You can retrieve the technical stuff (server, port) from tools>account settings>server settings. I just reviewed steps at Protonmail and they are quite explicit on the choices and the data required. Here is text I copied from one page, but there is much more there:

"Then log into the Import-Export app using your ProtonMail username and password.

Next, click on your username to reveal a list of your email addresses. Select the email address where you want to import the emails by clicking on the “Import” button to the right of the address.

Then you are given two choices: “Import from account” or “Import local files”. If you want to import emails from another cloud email service, such as Gmail or Yahoo!, click on “Import from account”. If you want to import emails from files stored locally on your device, including from an email client, such as Apple Mail, Outlook, or Thunderbird, then click on “Import local files”."

I do not mean to ignore your request, but protonmail documentation is quite good on the steps. Go to their site, click the support link and then look for the import support.

Thanks again David but if you go further there are no specific instructions to handle TB. FYI when I moved folders from Macmail to TB I exported the files in MBOX to my desktop and then imported them into my TB local folders. I was able to do that fairly easily with help. Frankly I don't recall exactly how I did it but it seems to me that this could be a way to do it this time. Again FYI I followed the steps on Protonmail after I sent the second message but the platform didn't work when I input my Mailfence email address and password. The server address and port # did not "automatically fill" as promised. Even if it did work I still wonder how the platform would figure out the the folders I want to import to Protonmail are not on Mailfence but in a folder on TB.

Well, I took another look at the Protonmail "documentation steps" and tried to make sense of them. As I've mentioned before I have saved email folders on TB in which is the client for a Mailfence account. The saved emails are in a bunch of folders all contained in "Local Folders" in TB totaling less that 1 gig. I want to move those folders to a Protonmail account. I have downloaded the Protonmail import/export app and that gives me two options from which to choose on where the email folders I need are stored, from an account and from local files. I really don't know which one if correct in my case so I've tried both. When I try account I input my Mailfence email account name and password and then the server and server address and port # do not auto fill as I'm told to expect so that doesn't work. I've tried it several times with the same result. My VPN has bee turned off each time. When I try the "local files" options it opens a bunch of files on my machine and I don't where to look for what I need or how to execute so I'm stuck there too. David it's easy for you to say that the documentation on Protonmail is "quite good" but they simply aren't working for me. I made a mistake when I changed from gmail with macmail client (which worked perfectly for decades) to Mailfence with TB client. I don't have a computer background and have no interest in knowing what goes on under the hood. I have many other interests that this mistake is taking valuable time away from. If there is anybody out there who can help me complete this task in a fairly straightforward series of steps I will be eternally grateful.

On the two choices, it would be from account if your current account is IMAP (folders kept on server) and 'local files' if your account is POP (messages downloaded to PC and removed from server). On local files, they are in your profile, located under MAIL and 'Local Folders ' - I respect it is loss of your valuable time, but it would be even more of a loss of valuable time loss for someone with no investment in the outcome. I do wish you well and your best bet may be to contact protonmail for direct assistance, as I feel confident they have worked with Thunderbird before.