Hello,
I disagree with your analysis;
while the Apparmor profile (♥) will prevent tragic things like gpg key
stealing, please keep in mind that an attacker can access every Firefox
files, like cookies (stealing sessions), stored passwords, changing
preferences (remember
http://net.ipcalf.com/ ?), executing code inside
the browser, …
This seems pretty serious to me, since people expect the web-browser to
be reasonably trustworthy.