Running Zimbra 5.0 (OSS) the "Next Page" arrow goes off the right-hand side of the screen and isn't accessible on the Mail pane. It seems that it gets pushed off the end by the number of other buttons on the toolbar.
As a workaround, using the right cursor does the trick, but it isn't ideal. Also the numbering (e.g. 1-25) isn't visible.
This is on Firefox 2 and 3 running on Ubuntu Linux at 1024 x 768 resolution.