I've never run into any
shell: links before, but you never know. You can
click here to see if you're vulnerable. (it'll go to shell.com if you aren't) Otherwise grab
this XPI and restart your browser after it installs. More info on
MozillaZine and
Full Disclosure. I sent that shell: link above over MSN and it automagically converted it to a link, which IE would then execute (if IE is your default browser), kinda funny.