Steps to reproduce

1. run the attached example in Firefox (testing with 42) with the setting below:

   <library-property>
       <name>org.zkoss.zkex.ui.comet.smartconnection.disabled</name>
       <value>false</value>
     </library-property>	
   

2. open 6 tabs for the same zul to reach the max concurrent connection per host
3. click the button start a long operation in one tab

Current result

the AU request is held by the firefox and is not sent out.
The "processing" message keep showing on the page.

expected result

AU request is sent out.

Debug info

1. in 7.0.7 and 8.0.1.1, chrome doesn't have such issue.
2. in 8.0.2.FL.20160325-Eval, chrome also has such issue.