How to open local HTML from a Thunderbird add-on preference pane?

Question

My Thunderbird add-on contains a help.html file, which can be opened from the preferences pane. This is the code:

  <label value="Open help window" 
 onclick="window.open('chrome://foo/locale/help.html', 'Help', 'chrome');" 
 class="text-link"/>

Under Windows, the HTML is displayed fine, but under MacOS, the background is transparent. Also, the link only opens once.

How do I properly open a local text from a Thunderbird preference pane? My add-has no other window (it is a contextual menu).

Thank you.