I was trying to fix the TinyMCE for joomla 1.5 in all ways. None worked. I even went into the core files and tried to change them. Finally I found the sloution by adding those lines to my template CSS

.mceToolbarTop * {
float:left;
}

.mceToolbarTop select {
width:auto!important;
}

.mceToolbarTop option {
float:none;
}