Firefox doesn't natively read .eml files, so it likely hands off the file to the program designated in the registry as the .eml handler. Are your other browsers doing that as well?

In the case of Firefox, does it make any difference whether Windows Mail is already open when Firefox tries to hand off the file?

If you save the file instead of opening it, is there any problem with the file name suggested?

When Windows Mail is already open it makes no difference.

I don't know how the other browsers do it. It are IE, Google Chrome, Opera, Safari and Maxthon 3. But it works correct with hem.

The other browsers don't open the Window Mail programm, only the mail is opened Firefox opens the Mail programm and not the mail its self.

I'm trying to think of a way to test this without access to your system. Any suggestions?

Just download an .eml file to FireFox!

see : https://support.mozilla.org/en-US/questions/754466

thank you

Just download an .eml file to FireFox!

This is not a file type I see every day, and for closest comparison, I would like one "served" in the same manner you are serving yours.

I have made .mht instead of .eml for the extension and the contenttype: message/rfc822.

This is a nice workaround for the problem, because Firefox don't support .eml.

Since when is Firefox supporting MHT, without installing the UnMHT or MAF extensions?

Correct, but this gives you the possibility to open the e-Mail in an other browser or programm (f.i. word or so)