Conkeror is able to invoke an external editor for editing web page text fields. XULRunner does not have direct support for complex interactions with external programs, so to support such a feature, Conkeror needs a special utility called conkeror-spawn-helper.
Important Note: currently, conkeror-spawn-helper only works on unix-like operating systems. We are actively clicking our heels wishing for a Windows-using hacker to add Windows support. In the meantime, it does work compiled with Cygwin. Run gcc conkeror-spawn-helper.c and patch spawn-process.js to include the .exe extension on the end of spawn_process_helper_program. Any program you spawn with spawn-process or spawn-and-wait-for-process will also need the .exe extension in the name.
If you installed Conkeror via your operating system's package manager, conkeror-spawn-helper is likely either already installed or available as a separate package. For example, on Debian and Ubuntu, this utility is available in the package conkeror-spawn-process-helper.
If you installed Conkeror manually, you need to run make in the Conkeror base directory. To do this, you will need to have the normal C compilation toolchain (like gcc, etc) installed on your computer. There is no need for a make install step because Conkeror will look in its own directory for the conkeror-spawn-helper executable.