Currently it's not possible to make the following breakdown in ReportBuilder: Mobile devices with the breakdown browsers (metric visits).
It is available in SiteCatalyst en Ad hoc analysis.
According to ClientCare this isn't a bug.
This breakdown is now available in Adobe ReportBuilder.
You can download the latest version by login into Adobe Analytics, and accessing the latest binaries from the Adobe Report Builder Download page.
This is actually possible in all tools in Adobe Analytics now. We added the ability to break down devices by browsers in the May 2014 release.