I did some more digging and finally figured out what we were doing to tickle this error in Firefox. Short story: passing a null to dojo.getComputedStyle(). In particular, we were making calls like dojo.marginBox(dojo.byId("a-bad-id")) which in turn calls into dojo.getComputedStyle() to do it’s work, and hence, the error. I updated the repro case and the original blog post to reflect the findings.

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