Tiny MCE line breaks when starting to write

Tiny MCE line breaks when starting to write

I'm using tinymce, and when I start writing inside the editor the line
breaks. I'm using explorer 9, but when I tested it with chrome it worked
fine and the line did not break!
what could be the problem? and how can I solve it ?
This is my code:
tinyMCE.init({
mode: "textareas",
theme: "advanced",
editor_selector: editor_selector_,
directionality: direction_,
theme_advanced_buttons1:
"cut,copy,paste,|,bold,italic,underline,strikethrough,|,undo,redo,|,justifyleft,justifycenter,justifyright,justifyfull,|formatselect,fontselect,fontsizeselect,|,forecolor,backcolor",
theme_advanced_buttons2: "",
theme_advanced_buttons3: "",
theme_advanced_font_sizes: "14px,18px,24px,36px",
theme_advanced_toolbar_location: "top",
theme_advanced_toolbar_align: align_,
theme_advanced_statusbar_location: "",
height: "225",
readonly: read_only_,
content_css : "resources/css/tinymceStyle.css" ,
setup: function(ed)
{
ed.onClick.add(function(ed, e)
{
saveGridState('editor');
});
}
});
Thank's In Advance.