Added date checking to fireplace (bug 1044230)

> IIUC, FxOS < 2.0 will use the old iframed app. FxOS >= 2.0 will use the packaged app where this is important. Would it be better to do a UA check and only insert this if FxOS > 2.0?

that doesn't make sense to me. if this code gets run, it'll get shown. if it doesn't get run, then the user is probably using an out-of-date version, like an older yulelog package.