Since the update to Ubuntu/Kubuntu Maverick Meerkat I was a bit puzzled why Thunderbird refused to open links in mails with my default browser Firefox even though it was set in the KDE settings and registered as a protocol-handler for http and https in the Thunderbird settings. It seems that there must be another setting in Thunderbird to use the default browser selected for the freedesktop environment which is now used. So updating the alternatives helped in Ubuntu:

sudo update-alternatives --config x-www-browser