We've just set the en-US defaults for mailto and calendar protocols for the upcoming major release of Firefox (release 3.0). They are being tracked in bug 413630.
The defaults will be:
mailto: Gmail, YahooMail, Windows Live Hotmail
calendar: Google Calendar, Yahoo Calendar, 30 boxes.
These should work internationally but of course for each locale we'll create and track bugs for changes on these defaults.
In doing the research and gathering some feedback from localizers worldwide there seems to be more choice for local webmail providers than web calendar providers. I guess the localization of calendar is not yet that popular.
If you feel we may have missed something major in the choice of these defaults, I'm open for feedback. :) And, as always, we can make changes for locales that offer local language and good user choice options.