Install ImportExportTools, and save multiple messages or folders in various formats, such as plain text, eml, html, pdf, mbox etc. A folder of messages can be saved to a single text file.

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

I did not find a version compatible Add-on to help me export multiple emails to separate text files. Are you familiar with a specific AddOn that will do this?

Most add-ons in this category are not compatible with the current version of Thunderbird, or export Contacts, not emails.

Note: I don't want to merge 200+ messages in a single text file: the goal is to have each year of minutes in a separate folder, with each meeting record in a file named by the meeting date, then to publish these minutes on Google Drive.

So now I am looking for a good free tool to convert .eml files to html or text!

The current version of ImportExportTools mentioned in my first reply is compatible with TB 14 - 59. Versions for older TB are available on the add-on website.

You can view eml files in any text editor if you prefer that approach.

OK, got it now, and it seems to do the job very conveniently; I think it will work for me.

I am not familiar with the interface for searching & installing AddOns, and find it awkward. My original search for ImportExport did not match the actual tool but came up with various things that did not seem useful... an exact match worked, but I found the whole interface confusing: must be really getting old! :-(