formards mails inline /attachments

Hello
I've just installing mozilla 1.8b5 (the problem with the 1.7.5) and I'm 
unable to forward a mail inline (and not in attachment).
At home I've an old mozilla version, and in mail properties I've the 
opportunity to specify inline or attachments for forward mails.

So anybody know how to re activate it ?
I've try to change something in the prefs.js, but nothings change and 
all what I do is deleted (in prefs.js).

So please help me.

Thanks in advance
Vincent