We use negative margins for correct positioning, but Firefox has a rendering bug when there's a negative margin at the end of an inline area. See bbc#21 (pull request) for details of the issue and the workaround.

https://bugzilla.mozilla.org/show_bug.cgi?id=1502610 is the Firefox bug ticket.

We can apply the same workaround as in the BBC fork here, at the risk that there may be small positioning errors.