I experience with IE9 (IE6, IE7, IE8, Firefox3./4. work fine!) the following problem with tabs (ActiveWidgets version 2.5.6).
Sometimes (unfortunately not always consistently) on load of the page, only the selected tab is shown in the tab. The border line is expanded to the maximum width, the background image is missing and the other tabs are hidden.
Only if I click with the mouse on the selected tab, the tab bar is refreshed correctly and all tabs are correctly displayed.