Firefox -> Entry removes character 's' during typing

In my firefox on a Solaris 10 Box I've configured
Freemono fontsize 12
for the Monospace font.

During the input in a textfield when a 's' character appears in the input,
it magically disappears and seems, as if the 's' character is missing ( but it
is there ).
Changing from freemono to monospace let's the 's' also reappear.