The signature is created with a HTML CSS class .moz-signature and you
can add style information for it to make it use a different font or make
other visual changes (like suppress the opaque look that it has), but
unfortunately it appears that many other mail programs ignore the inline
style information that you need to insert for that.
(it has to be in the head section but in this case it is in the body
section, thunderbird/seamonkey still acts upon it but microsoft outlook
will ignore it)
The problem you mention is indeed very apparent. There should be some
option to make the signature read-only, so one cannot inadvertently type
over it, delete part of it, etc.