If someone is interested :
you must modify \wp-content\themes\customizr\parts\class-header-menu.php
I’ve let the menu for IE 9 with that :
<?php if (strstr($_SERVER["HTTP_USER_AGENT"], "MSIE 9.")){//IF IE 9
wp_nav_menu( array( 'theme_location' => 'main' , 'menu_class' => 'nav' , 'fallback_cb' => array( $this , 'tc_link_to_menu_editor' ), 'walker' => tc__ ( 'header' , 'nav_walker' )) ); // write the menu
}else{
//else write the short button
?>
<button type="button" class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<?php } ?>