Netresearch PaymentFee

Open app/code/community/Netresearch/PaymentFee/etc/config.xml

and find the following code:

<controller_action_predispatch_checkout_onepage_savePayment>
    <observers>
        <paymentfee>
            <type>singleton</type>
            <class>paymentfee/observer</class>
            <method>handlePaymentFee</method>
        </paymentfee>
    </observers>
</controller_action_predispatch_checkout_onepage_savePayment>

Replace it with:

<controller_action_predispatch_checkout_onepage_savePayment>
    <observers>
        <paymentfee>
            <type>singleton</type>
            <class>paymentfee/observer</class>
            <method>handlePaymentFee</method>
        </paymentfee>
    </observers>
</controller_action_predispatch_checkout_onepage_savePayment>
<controller_action_predispatch_firecheckout_index_savePayment>
    <observers>
        <paymentfee>
            <type>singleton</type>
            <class>paymentfee/observer</class>
            <method>handlePaymentFee</method>
        </paymentfee>
    </observers>
</controller_action_predispatch_firecheckout_index_savePayment>

Save the changes and clear the cache.