Once exported there are no .emlx files. I do not understand what is being claimed…
Do you want to convert Mac Mail files to Thunderbird file format? I suggest the best conversion process is called Mailvita EML/EMLX to MBOX Converter Software. This application supports all email clients of the EML file, such as Windows Live Mail, Mac Mail, Outlook Express, Apple Mail, etc. You can convert bulk EMLX files to Thunderbird MBOX file format without any errors. This tool has a preview option before the conversion. Supports all Mac and Windows OS versions. Download the free demo version of the utility.
Here is the output I got from using Export Mailbox... from the right clicking on a folder in Apple Mail.app:
% find .
.
./Fedora
./Fedora/RPM Fusion Devel.mbox
./Fedora/RPM Fusion Devel.mbox/mbox
./Fedora/RPM Fusion Devel.mbox/table_of_contents
./Fedora/RPM Fusion Compose.mbox
./Fedora/RPM Fusion Compose.mbox/mbox
./Fedora/RPM Fusion Compose.mbox/table_of_contents
./Fedora/Systemd.mbox
./Fedora/Systemd.mbox/mbox
./Fedora/Systemd.mbox/table_of_contents
You can see there are no odd types of file and an obvious mbox file to import into your mailer of choose on Fedora.
Given Mail.app can do the export I am not sure why people keep proposing convertor apps.
I just tested the exporeted mbox files with thunderbird.
They just work, but its a bit fiddle to find out how to import them.
I read this: Import MBOX to T-bird 115 | Thunderbird Support Forum | Mozilla Support
It points out that all that you need to do is copy the mbox file into
the ~/.thunderbird/<profile>.default-release/Mail/Local Folders
folder with the name you want to mbox to have thunderbird.
To find the magic profile look in ~/.thunderbird:
$ ll ~/.thunderbird/
total 12
drwx------. 1 barry 1510 2024-02-23 08:26:56 76y5tuwm.default-release/
drwx------. 1 barry 20 2024-02-23 08:11:25 f64b59ka.default/
-rw-r--r--. 1 barry 62 2024-02-23 08:11:25 installs.ini
-rw-r--r--. 1 barry 259 2024-02-23 08:11:25 profiles.ini
For my test I have exported a SystemD folder from Apple Mail.app.
I use scp to copy the file to my test Fedora VM.
I did this to add the mbox:
$ mv mbox ~/.thunderbird/76y5tuwm.default-release/Mail/Local\ Folders/Systemd
$ ls -l ~/.thunderbird/76y5tuwm.default-release/Mail/Local\ Folders
total 4872
-rw-r--r--. 1 barry barry 25 Feb 23 08:14 msgFilterRules.dat
-rw-r--r--. 1 barry barry 4763587 Feb 23 08:22 Systemd
-rw-------. 1 barry barry 0 Feb 23 08:21 Testing
-rw-r--r--. 1 barry barry 1301 Feb 23 08:21 Testing.msf
-rw-------. 1 barry barry 0 Feb 23 08:13 Trash
-rw-r--r--. 1 barry barry 1547 Feb 23 08:19 Trash.msf
-rw-------. 1 barry barry 0 Feb 23 08:13 'Unsent Messages'
-rw-r--r--. 1 barry barry 1368 Feb 23 08:26 'Unsent Messages.msf'
Thanks Barry. You made two posts. The first I understood and agree, if that works, I may not need any convertor tools. but your second post went into the weeds for me, not a clue what you’re doing, or how, or even on which OS (linux or Mac!)
I am not up to using CLI for this, but your first post may still work. I am going to do a test export of just one of my local folders (which has 1 or 2 subfolders inside) and see if I can get those to import onto Thunderbird on Mac, I thin that’s simplest. Not sure, but think so. if that works, I can delete, and do a new export of all my 40-50GB of local folders!
One of the big issues is maintaining the structure. I MUST retain the structure of my folders, there are many, and they have many more nested inside. Doing this by hand in CLI (even I WAS up to that task!) would be a pretty big headache for a seasoned pro, which I am most definitely not 
Will see what I can do with GUI tools in Mac Mail and TB, and report back. thanks
PS being very short of money I do agree that it would be much nicer to avoid 3rd party tools which are damn expensive for what they do and I don’t have cash to spare, so whilst it’s an option, it’s the very last on my list for now!
PS - Let’s just say I manage to export Mac Mail into a nice MBOX file/set of files, and manage to import them into Thunderbird ON MAC.
Do you know if I could drag and drop those in GUI on Linux, to move my TB data from Mac to Linux?
Worst case I could very carefully paste some CLI commands, but it scares me a bit, and if the file manager can allow me to drop them all, that would easier for me. Is that possible do you know?
thanks
I would copy the mbox files to fedora and import into thunderbird there.
I think using thunderbird on mac will add complecations.
That 2nd post that comfused you has the detail of how to import into thunderbird. You can use a gui file manager instead of the cli commands.
Use the folder paths from the commands I used to figure out where to lok with the gui.
Note: thunderbird does not have a gui import command for mbox files.
It appears that those in the know are sure it wont work, “it” being for me to export all my local folders to MBOX files from Mac Mail, then just ask Thunderbird to import them. Apparently it will pull it in, but wont retain directory structure, making a God awful mess. This refers - Do I need this? · Issue #536 · thunderbird/import-export-tools-ng · GitHub
What appears to be true is that you will have to rebuild the tree of mailboxes.
There is no way I saw of adding all the mbox files in one go in the tree that was in mac mail.
But you only have to do the move once.
I assume that you will be able to move the mail folders around in thunderbird once you add them side-by-side.