Why Thunderbid does not let me change the default browser easily?

If I have already set my default browser to google-chrome in the system settings of my newly installed Kubuntu 15.04, why does Thunderbird not accept it?
Why do I have to manually type strings like xdg-mime query default x-scheme-handler/http to see that the default browser is still firefox.desktop?

So lets submit xdg-mime default google-chrome.desktop x-scheme-handler/http to update the default browser.

Thank you for pointing this out at http://kevinlocke.name/bits/2012/07/18/thunderbird-default-browser-linux/.

Previously, I had edited my Thunderbird setting while adding network.protocol-handler.app.http, my system settings with application/x-extension-html, application/x-extension-shtml, application/x-extension-xhtml, application/xhtml+xml, text/html and checked my mimeTypes.rdf in the profile folder.

Well, in the end it worked…