Ask for a long max-age on the iframes, cachebust it with the build id (bug 1087399)

https://bugzilla.mozilla.org/show_bug.cgi?id=1087399
actually, ignore me. they both have the same effect. and per the [HTTP/1.1 Spec](http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html):

> The max-age directive takes priority over Expires, so if max-age is present in a response, the calculation is simply:
>
>     freshness_lifetime = max_age_value

so, don't change a thing - just address the query-string issue and add a comment, please.