Go to the Run/External Tools/External Tools ..." menu item on the "Run" menu bar. Select "Program" and click the "New" button. On the "Main" tab, fill in the "Location:" as the full path to your "mvn" executable. For the "Working Directory:" select the workspace that matches your webapp. For "Arguments:" add jetty6:run.
Move to the "Environment" tab and click the "New" button to add a new variable named MAVEN_OPTS with the value:
-Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,address=4000,server=y,suspend=y
If you supply suspend=n instead of suspend=y you can start immediately without running the debugger and launch the debugger at anytime you really wish to debug.
Then, pull up the "Run/Debug/Debug ..." menu item and select "Remote Java Application" and click the "New" button. Fill in the dialog by selecting your webapp project for the "Project:" field, and ensure you are using the same port number as you specified in the address= property above.
Now all you need to do is to Run/External Tools and select the name of the maven tool setup you created in step 1 to start the plugin and then Run/Debug and select the name of the debug setup you setup in step2.