See the testcase. It tries to get the computed value of display property on a pseudo-element of a flexbox. The pseudo-element doesn't really exist because it has no content. Stylo shows "inline" in this testcase (so does Blink), while Gecko shows "block". From the spec, it seems that it should return "block" because blockification applies as far as the computed display value is not "none" or "contents", and absence of content property doesn't compute the display value to anything different. Web-platform test cssom/getComputedStyle-pseudo.html checks this behavior. I don't think it is an important issue, though.
You need to log in before you can comment on or make changes to this bug.