Which are the registry settings to set Firefox an Thunderbird as defaults?
It can be done with any script or little program or firefox/thunderbird command line?
The target is do it for all the users on a computer not just the current.
There is no easy way to set the browser default for all users on a computer. The procedure is slightly different for Windows XP and Vista and above so you would need to specify your questions with the operating system, but maybe the following is enough for you:
For firefox you could just run "firefox.exe -silent -setDefaultBrowser" on the command line for each user. For Thunderbird there is no simple command like that, but you could try the utilities suggested in (1), which do the work for you, but again on a per-user basis. Does this help your?
The registry settings you would need to change are all in HKEY_CLASSES_ROOT and you can find a summary in (1) - but I would advise against a script changing these entries (HTTP and HTML etc.) manually because the built in function from Windows "Set Program Access and Computer Defaults" does everything perfectly.