as thunderbird-0.8 sets (by registering itself as the default mail application) and uses the /desktop/gnome/url-handlers/mailto gconf key (which value is /usr/lib/MozillaThunderbird/thunderbird "%s"), /usr/bin/thunderbird isn't used and the already existing instance of thunderbird (if there is one) isn't detected when clicking on a mailto: link from firefox-1.0pr, and the profiles dialog box is shown. tested with mozilla-launcher-1.20.
closing, at it works now using mozilla-firefox-1.0.4, mozilla-thunderbird-1.0.2 and mozilla-launcher-1.32.