linebreak <br /> changes to <br> automatically

Hello,

I'm searching for hours for a solution. Now I hope someone can help me.

I'm using ckeditor with no security filters and a richtext text format (all hmtl-tags are allowed).
I can see in the ckeditor code-view all line-breaks as
. But unfortunately after saving I see in firebug the
-tag was changed to
.

Thus in older firefox browser these
tags are shown as strange characters in some of my blocks.

Any ideas?

Thanks