UTF-8 in window title

Hello everyone,

I like to use a string containing UTF-8 characters as a window title for 
my XUL application:

   document.title = whatever;

But the title just shows question marks for those characters which are 
not ASCII compatible. I saw that Thunderbird's mail editor is also using 
the title setter in the same way, but has no problem displaying UTF-8 
characters.

Do I need to do more? Is it maybe a font problem? I appreciate any tipps 
and hints.

Thanks,

Daniel