If you select invoices and scroll down, the options - Summary, Quote, Orders, Invoices, Credit Notes and Delivery Notes scrolls off screen. So you always have to scroll back up if you want to view another tab. This is also an issue in the Main Manager program - but is more of an issue in the Customer Portal because customers would not be familiar with Manager, so the tabs really need to be always visible.
Given that there are only six tabs maximum that can be displayed on the Customer Portal, it would make more sense to place the tabs in the same row as the name of the client and have this row and the table header rows frozen so you always see the tabs, table name such as invoices and header rows such as Issue Date, Reference and Description. Unless more many more tabs will be introduced for the Customer Portal, I think having the tabs horizontally rather than vertically would make sense.
As for the Main Manager program, if the table header rows could be frozen as well that would be a massive improvement as by design these should always be visible.