It is possible for page in domain A in possession of a window handle of frame / window in domain B, to set arbitrary non-existent location.* values, for example: third_party_window.location.foo = 'whatever'.
This is probably an unnecessary side channel, and could theoretically become an attack vector against any site that relies upon a location.* property or method that is not implemented in Firefox but supported elsewhere (and hence would not be protected), or simply got mistyped.
This behavior follows from location being defined as allAccess as opposed to other similar properties (like history) where only the getter is allAccess.
This is presumably because in addition to needing to _get_ it to be able to set its properties like .hash and .href, we can also legitimately _set_ location's value directly.
This is fixed on the trunk by Cross-origin wrappers (XOW). There are still regressions we're hammering out but we'd eventually like to land XOW on the 1.8 branch as well to stop issues like this.
Off the top of my head I'm not overly worried about sites using this as a communication channel--there are other ways they could cooperate if they wanted, either server-to-server, using Firefox3's cross-site XHR, funky url-parameter tricks, etc. Without XOW, though, this can be leveraged into a XSS attack (see bug 361961).
Given that we're implementing postMessage() on the trunk we're probably not too concerned about the information-passing aspect of this. The fact that the property can be any doctored object, though, could lead to nasty surprises on sites who were storing things on the Location object. Does anyone actually do that? Hixie could do a search perhaps.
This was fixed by XOW (bug 367911).