Hi František,
I am experimenting with this a bit myself using jpm 1.0.1[1] and I can confirm this works in Earlybird 39.0a2 (2015-04-24) on Ubuntu (as well as in Firefox).
The bigger issue is that the high-level add-on SDK model concept of add-on scripts and content-scripts does not apply to thunderbird because it is still a XUL application.
You basically have to wedge in via require('chrome') to get access to its interfaces.
See also
https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tutorial/Adding_HTML_Elementswhich I have succeeded in using to add a combination of HTML and XUL nodes to existing thunderbird windows.
I only experiment with this to possibly get some functionality into thunderbird I plan to add to Firefox as a add-on.
Hope this helps,
Adrian