Tested 8/18, Firefox correctly handles a simple download
attribute. The accepted and second-ranked answers are wrong, at least as of now.
So, simply,
<a href="http://test.com/path/to/my/file.mp3" download></a>
will work, and if you want to control what the file is downloaded as, you give download
a value:
<a href="http://test.com/path/to/my/file.mp3" download="use-this-name.mp3"></a>
That is why Chrome (or any modern browser) will download it as "true.mp3" if you try to use download="true"
instead of a simple download
to force the file download.
Also, note that <a></html>
in the question isn't valid, and could have possibly caused a problem at the time if that's not just a typo.