While you're clearly NotAComputerGuy that shouldn't keep you from trying as past failures are not a guarantee for future failures :-]
Read, practice, ask, learn.

You're using the Debian edition of Mint. Try looking for Icedove. Same program, different name due to licensing issues.

I also want to point out, IIRC as it's been a while since I last used Thunderbird, that it's native file format is (was) in a mbox-like format. So, no "importing" should be required. Just copy the mbox format file into where TB/Icedove keeps its mail files, giving the file the name of the "folder" as you wish it to appear in TB/Icedove.

I just download them with POP3 (deleting them from the server once they are downloaded, so they do not take up space) using fetchmail + procmail. They are downloaded as plain text files and you can automatically sort them in directories. And you could set up an encrypted directory for mail using encfs -- this directory grows to accomodate whatever you put in it, so you don't have to allocate a certain amount of disk space beforehand.

As for the format: I have no idea what .eml is, but there is an e-mail format which looks something like this:

I know very little about how e-mail works, but I believe that is the standard format (which is plain text -- even the attachments are sent as plain text, converted from binary to ASCII and simply appended to the text file). All of the text and attachments are encoded in order to be sendable as e-mail, so it has to be decoded before it can be read, but MUAs like mutt do that for you.

I was going to say that it works just like webmail, but it's better because you can set up a script to check for new e-mails automatically every 10 minutes, and show a little notification (like xbiff) when you have new mail.

.eml files will open with Outlook / Outlook Express in windows. You can view an .eml file and you'll see it's nothing but the complete email message stored as plain text.

In thunderbird on windows, you can drag messages out of tbird to a folder and they are saved as .eml. I don't recall if this is the default thunderbird behavior or if it's happening because I installed the import & export add-on.