While the class names of rounded elements may change in the future, you can style the current version of the menu to have square corners instead of rounded corners in all browsers.

<style type="text/css">
.a2a_menu, .a2a_menu_inside, .a2a_menu a, .a2a_tabs div, .a2a_menu_find_container, .a2a_menu_powered_by { -webkit-border-radius: 0 !important; -moz-border-radius: 0 !important; border-radius: 0 !important; }
</style>