At https://esdiscuss.org/topic/tostringtag-spoofing-for-null-and-undefined#content-59 Allen suggests a reliable brand test to replace most uses of Object.prototype.toString.call, which will no longer be a reliable brand test as of ES6. https://codereview.appspot.com/198470043/ is a draft update of SES to use the new technique. (Once submitted I will post a better link.) However, testing across browsers reveals that this isn't reliable on FF35.0.1. For all exotics tested at this time -- Date, Number, Boolean, String, RegExp, WeakMap -- a proxy with such an exotic as target passes the same brand test. This problem is fixed as of FF Nightly 38.0a1, so this bug can be immediately marked as fixed. I don't know its status on FFs between 35 and 38.

This was actually fixed by bug 1111243. Good to know this actually a positive change for you.