Capsicum is a lightweight OS capability and sandbox framework provided
by FreeBSD.  When built with Capsicum support - which is the default under
FreeBSD - Irssi can enter a Capsicum capability mode (a sandbox), greatly
limiting possible consequences of a potential security hole in Irssi
or the libraries it depends on.

To make Irssi enter capability mode on startup, add

capsicum = "yes";
awaylog_file = "~/irclogs/away.log";

to your ~/.irssi/config and restart the client.  Alternatively you can
enter it "by hand", using the "/capsicum enter" command.  From the security
point of view it's strongly preferable to use the former method, to avoid
establishing connections without the sandbox protection; the "/capsicum"
command is only intended for experimentation, and in cases where you need
to do something that's not possible in capability mode - run scripts,
for example - before continuing.

There is no way to leave the capability mode, apart from exiting Irssi.
When running in capability mode, there are certain restrictions - Irssi
won't be able to access any files outside the directory pointed to by
capsicum_irclogs_path (which defaults to ~/irclogs/).  If you change
the path when already in capability mode it won't be effective until
you restart Irssi.  Capability mode also makes it impossible to use
the "/save" command.

Currently there is no way to use custom SSL certificates.  As a workaround
you can establish connections and enter the capability mode afterwards
using the "/capsicum enter" command.