I don't think there is a 'select all' option when you select mails, at least there isn't in the latest (3.0) mail app. If there is no bug report in the Mozilla bugtracker, you could add it as a feature request.

You should be able to delete the trash via webinterface of your imap/email provider though.

Thank you acidicX this confirms what i feared. Btw it's a pop mail account.