I have implemented the virtual scrolling using the kendo combobox in my mvc project and when I ran it on firefox version 39.0.3 it worked very well. But when i open the list and scroll using mouse wheel or click on the scroll bar down arrow then after few items the selection goes back to the first item. So i tried to open your example site using this link http://demos.telerik.com/aspnet-mvc/combobox/virtualization and it behaved in the same way in firefox. So can you please provide me the solution to fix this issue.
Here are the steps to reproduce it.
1) Open this http://demos.telerik.com/aspnet-mvc/combobox/virtualization link in firefox
2) Click on the drop arrow to see the list of items in the drowdown
2) Scroll using mouse wheel or click on the scroll bar down arrow. after few records the selection will jump again to first item.