Using custom.phtml template
This feature is available since Firecheckout 3.4.0
To add additional php code at firecheckout page, you can create custom.phtml
file at app/design/frontend/[PACKAGE/THEME]/template/tm/firecheckout folder.
The file will be automatically included at the firecheckout page at the bottom of content column.
Please note, that this file could be used for php and javascript code only. Do not use it for visible content