nextpleaseOptions.css

tabbox.vert-tabs {
    -moz-box-orient: horizontal !important;
    border: 5 !important;
}

tabs.vert-tabs {
    -moz-box-orient: vertical !important;
    background: red;
}

part of nextpleaseOptions.xul

<tabbox flex="1" class="vert-tabs">
                    <tabs class="vert-tabs">
                        <tab label="&options.next;" class="vert-tabs"/>
                        <tab label="&options.prev;" class="vert-tabs"/>
                        <tab label="&options.first;" class="vert-tabs"/>
                        <tab label="&options.last;" class="vert-tabs"/>
                    </tabs>
                    <tabpanels flex="1" class="vert-tabs">
                        <tabpanel flex="1" class="vert-tabs">
                            <vbox align="left" flex="1">
                                <listbox id="nextfields" seltype="multiple"/>
                                <hbox align="center" flex="1">
                                    <textbox id="addnextfield" type="text" flex="1" maxlength="256" />
                                    <button id="addnextbutton" class="dialog" label="&options.add;"
                                        oncommand="nextplease.addToListbox('addnextfield', 'nextfields'); return false;" />
                                </hbox>
                                <hbox>
                                    <spacer flex="1" />
                                    <button class="dialog" label="&options.removeSelected;"
                                        oncommand="nextplease.removeSelectedFromListbox('nextfields'); return false;" />
                                </hbox>
                            </vbox>
                        </tabpanel>
                        <tabpanel flex="1" class="vert-tabs">
                            <vbox align="left" flex="1">
                                <listbox id="prevfields" seltype="multiple"/>
                                <hbox align="center" flex="1">
                                    <textbox id="addprevfield" type="text" flex="1" maxlength="256" />
                                    <button id="addprevbutton" class="dialog" label="&options.add;"
                                        oncommand="nextplease.addToListbox('addprevfield', 'prevfields'); return false;" />
                                </hbox>
                                <hbox>
                                    <spacer flex="1" />
                                    <button class="dialog" label="&options.removeSelected;"
                                        oncommand="nextplease.removeSelectedFromListbox('prevfields'); return false;" />
                                </hbox>
                            </vbox>
                        </tabpanel>
                        <tabpanel flex="1" class="vert-tabs">
                            <vbox align="left" flex="1">
                                <listbox id="firstfields" seltype="multiple"/>
                                <hbox align="center" flex="1">
                                    <textbox id="addfirstfield" type="text" flex="1" maxlength="256" />
                                    <button id="addfirstbutton" class="dialog" label="&options.add;"
                                        oncommand="nextplease.addToListbox('addfirstfield', 'firstfields'); return false;" />
                                </hbox>
                                <hbox>
                                    <spacer flex="1" />
                                    <button class="dialog" label="&options.removeSelected;"
                                        oncommand="nextplease.removeSelectedFromListbox('firstfields'); return false;" />
                                </hbox>
                            </vbox>
                        </tabpanel>
                        <tabpanel flex="1" class="vert-tabs">
                            <vbox align="left" flex="1">
                                <listbox id="lastfields" seltype="multiple"/>
                                <hbox align="center" flex="1">
                                    <textbox id="addlastfield" type="text" flex="1" maxlength="256" />
                                    <button id="addlastbutton" class="dialog" label="&options.add;"
                                        oncommand="nextplease.addToListbox('addlastfield', 'lastfields'); return false;" />
                                </hbox>
                                <hbox>
                                    <spacer flex="1" />
                                    <button class="dialog" label="&options.removeSelected;"
                                        oncommand="nextplease.removeSelectedFromListbox('lastfields'); return false;" />
                                </hbox>
                            </vbox>
                        </tabpanel>
                    </tabpanels>
                </tabbox>

Works fine, but looks quite bad, that is, which is why I tried to play around with CSS. See Tabs at http://www.hevanet.com/acorbin/xul/top.xul for an example.