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In Internet Explorer 6 the certificate is accepted by clicking on "Yes":
In Internet Explorer 8 you must click on the link "Continue to this website (not recommended)":
In Firefox you must first click on "Or you can add an exception...":
Next, click on "Add Exception":
Next, click on "Get Certificate":
Complete by clicking on "Confirm Security Exception"
If the warning is different, i.e. "Failed to validate certificate", read instructions here.
Internet-selainten sertifikaattivaroituksen hyväksyminen