Following the message of XenGeek
I used this code to remove Tabs keeping the title bar.

/* Remove horizontal tabs */
#tabbrowser-tabs {
visibility: hidden;
margin-top: 40px;
margin-bottom: -50px;
}

/* Remove sidebar header */
#sidebar-header {
visibility: collapse;
}

Anyway this add-on is very good, I don't understand why this tree tab menu is not a standard in Firefox. Saved me a lot of time and energy !

Thanks a lot !