gecko-sdk-1.7.12.ebuild has these use flags: mozcalendar mozdevelop moznocompose moznoirc moznomail. They are useless, since this is just the SDK and we only want the Gecko engine and the headers. Also, the ebuild itself doesn't regard them and just disables these things.
Yes, it has useless flags, the reason for this is that it is an exact copy of the mozilla ebuild except for the the install procedure. This is for maintainence purposes. Building a huge package like mozilla can be troublesome, using the ebuild from the mozilla maintainers makes this procedure much simpler. If the mozilla team wants to do it otherwise, feel free to chime in.
In this case it's simply a matter of removing those flags from the IUSE variable, I guess it can even be automatically removed by a script.
No, removing the use flags from IUSE is not an option because they are used in the ebuild. Portage will complain.
I'm going to close this as WONTFIX, feel free to reopen if you disagree.