For the client part it may help to just open the Firefox Error Console. (JavaScript errors)
If you want do debugging on the server part, you can see set the zimlet JSPs debug output when setting the log level for the zimelts to DEBUG.
Configure this the following way:
Add the following line to
/opt/zimbra/conf/log4j.properties.in:
Code:
log4j.logger.zimbra.zimlet=DEBUG
After editing log4j.properties.in you need to restart mailboxd:
zmmailboxdctl restart. Then watch logfile
/opt/zimbra/log/mailbox.log.
The Zimbra server does some caching. So after installing a new Zimlet or new Version its a good idea to restart mailboxd and to do a logoff/login.