zcs-5.0.10_GA_2638
I couldn't help but notice that external users have a horrible experience browsing briefcases and was wondering if there were plans to repair this problem.
I have reported it as a bug (#32782). If you create a Briefcase with multiple subdirectories then Zimbra will open a new window in IE if an external user clicks on a subdirectory.... and Firefox will open a new tab instead of keeping it all in one Window/Tab. I don't know if a lot of people simply don't use the external user option or what, but this is a major flaw that seems to have been overlooked.
So say you make Photos/Year/Place/image.jpg ... then each directory within the root Briefcase (Photos) will cause 2 extra tabs/windows to open in order to make it to image.jpg instead of simply browsing to the subdirectory in the same window.
Also, I have noticed that if you try to backtrack using the list located at the top of the external user web interface for Briefcases, it will also open an additional tab or window. Not only that, but in both browsers the root briefcase is not clickable in order to return to it.
If there is something I missed that changes this, then please let me know. I figured it would be set up to stay within one window by default but I may be wrong. It may be something that has an option somewhere to be fixed but after a couple days of looking I decided to give the developer forum a crack at it or at the very least make the developers aware of the issue. I know most of the stuff Zimbra is doing is rather groundbreaking and things take time to iron out.
If anybody has any suggestions other than creating an internal account then please let me know.
Thank you,
mace@rancorous.net
Bugzilla
Wiki
Source
External users Briefcase problem. 
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