After openeing the AJAX Control tool kit solution, go to Tabs.css under AjaxControlToolkit/Tabs/
either comment this line
.ajax__tab_default .ajax__tab_header {white-space:nowrap;}
or change it to
.ajax__tab_default .ajax__tab_header {}
Rebuild the site, and update the reference of AjaxToolkit in your project to the new one. You should see 2 rows like this