I have a few users with largish mailboxes (approx 20,000 messages), and they sometimes get warnings while running the Zimbra web client. A message that says the script is taking an unusually long time to run comes up and gives the user a chance to cancel it.
I think the 5 second default in most firefox installations is a bit too agressive, and moved the value up higher, to give the web client a little more time to run.
In firefox, go to the URL: about:config
and change the setting dom.max_script_run_time to 60. Not "60", just plain 60.
I will report how well this works. Unfortunately there is no way to configure this in internet explorer.
http://www.thescripts.com/forum/thread453578.html - place where I found out about the setting.