by debug mode do you mean:
debug output ?
you may configure your output to cosole/log in
attach a debugger to a runnig jboss?
you may configure the JPDA settings in
then attach your debugger to the address/port
copy the run.sh startup script to debug.sh (or whatever you want) and replace the the $JAVA_OPTS line in the file w/ this one:
JAVA_OPTS="$JAVA_OPTS -Dprogram.name=$PROGNAME -classic -Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,address=8787,server=y,suspend=y"
the above is taken directly out of the run.bat file. (make sure it is all on one line in the file).