Why would you want to "Save-as" an EML file again? Why don't you simply rename the file to whatever you want?

Stans: Have you done this before, just renaming file extensions? Do you know the difference that occurs between renaming to TXT versus "Save-as" TXT?

I ask rhetorically, because renaming the file extension does not create a readable document from an EML file. It is using "Save-as" to a HTML or TXT format using Thunderbird that creates a readable file.

Plus, for emails I choose to save for later viewing, using a TXT or HTML formats, provides a more simplified file format that is far more readable across more applications, which means easier accessibility.

Do you want me to go further into more reasons for saving emails to other file formats, especially when having no other choice to use the EML format to save from Windows 10 Mail APP, or is what I said sufficient?

Mark2012 said

Stans: Have you done this before, just renaming file extensions? Do you know the difference that occurs between renaming to TXT versus "Save-as" TXT?

As a matter of fact, I have. Mail files (.eml) are just normal plain text files. Changing the extension from .eml to .txt doesn't affect the contents of the file.

I ask rhetorically, because renaming the file extension does not create a readable document from an EML file. It is using "Save-as" to a HTML or TXT format using Thunderbird that creates a readable file. Plus, for emails I choose to save for later viewing, using a TXT or HTML formats, provides a more simplified file format that is far more readable across more applications, which means easier accessibility. Do you want me to go further into more reasons for saving emails to other file formats, especially when having no other choice to use the EML format to save from Windows 10 Mail APP, or is what I said sufficient?

If by "readable" you mean easier to read, then renaming the extension won't do. The eml file has extra code that can be a headache to comb through just to get to the contents of the message. I think I now understand why you want to re-save the contents of the eml files to a simple txt file. Well, the Thunderbird eml viewer has limited functionality as you've found out. Maybe the feature is there and needs to be unlocked, but I highly doubt it. I don't think converting between one format to another was part of the reader's intended use. Perhaps what you need is an eml to txt converter. I have no experience with such a tool, but a Google search returns several options, for both online and offline use.

Glad you understand why I asked the question.

When the EML file is opened "externally" from the Thunderbird program, the file still opens a Thunderbird UI view window, but the "Save-as" function does not work. Only when the EML file is opened inside the Thunderbird program, the "Save-as" function works.

Likely because there are DLL files that are only made active when the Thunderbird program itself opens.

Using 3rd party software This is not needed, because if this is the only option and there is no fix for Thunderbird, then it is just as easy to open Thunderbird and drag the external EML file into a blank account inside Thunderbird to use the "Save-as" function.

As you say: drag the external EML file into a blank account inside Thunderbird to use the "Save-as" function. This implies you have already deleted that email from Thunderbird, hence requiring to drag it back.

Initially, the Save as function is used to create the .eml file, but as it does not delete the original email at this point in time, you could immediately repeat the same 'Save as' function to create the text file and then delete the original from Thunderbird. Thus performing a double save as to get the two different types of files. This may be the best method to use going forward.

On eml files you currently have saved to desktop open Notepad open eml email 'View' > 'Message Body as' > 'Plain text' (This ensures you get all the link information)

Ctrl+A to highlight all text, then right click and select copy and paste into Notepad In eml select 'More > 'View Source' Copy just the 'Subject, From, To, Date' part in header and paste into Notepad above the previously copied text. Save the text file.

Not as simple as saving as text file at time of saving as eml file, but it works.

I agree that in an opened .eml file the 'File' > 'Save as' > 'File' should offer to save as a txt file. I've opened a bug report. https://bugzilla.mozilla.org/show_bug.cgi?id=1625447

It maybe helpful if others could say they needed this fixing and vote for the bug.