I'm not aware of a reason why the spacing should be the same. Noticed with changes for bug 1309086. https://treeherder.mozilla.org/logviewer.html#?job_id=28924850&repo=try#L5931 Test added in http://hg.mozilla.org/mozilla-central/rev/b4290a438134 with no bug number.

(In reply to Karl Tomlinson (:karlt) from comment #0) > I'm not aware of a reason why the spacing should be the same. Indeed, that's not a safe expectation. With simple fonts, it usually would be, but if the font in use has kerning or other positioning adjustments at that place, you'd get a discrepancy. (In principle, there could even be bigger differences -- entirely different shaping -- but that's unlikely except in a highly cursive or ornate font style.)

Too late for firefox 52, mass-wontfix.