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.