Hmm. It doesn't look good.

http://kb.mozillazine.org/Import_.MSG_files

I get the impression that the .PST file might have been more useful to you.

What happens if you double-click an MSG file?

Zenos, Thanks for the comment - I didn't want the multi-megabyte PST, just the emails relating to a boat cruise we are doing this weekend, the PST is now 100 miles away at home! The file you reference is from 2009 and is probably a bit out of date for Thunderbird 45.1.0, Windows 10 and Outlook 2013. I have found Free MSG Viewer which will serve my needs for now but will look at the msg to eml converters for future use. I come from a CAD background where almost all formats DWG, DXF, DGN and others can be happily opened by different programs. Surprising Thunderbird hasn't got a .msg import option yet as email is so essential and most companies use Outlook/Exchange.

Perhaps the fact MSG is a Microsoft proprietary format (real licenses and patents) you might understand no one here is interested in touching it. They chose to use a proprietary format when their was a perfectly good open standard (EML). For decades Microsoft have stood by their proprietary file formats and their customers continue to pony up with licensing fees for upgrades and ask the open source community to fix the product they are paying for. Like you they express surprise that the whole world does not use Microsoft formats.

The solution, and the only real solution is to purchase a product that converts the file formats.

BTW 2009 is very much relevant to today. The status of Microsoft's proprietary file format has not changed in the intervening years and is never likely to.

You could get a 30-day trial of Outlook 2016 (Office 365), but it's a lot of setup to go through...