Those should not be drawn by font. I believe the only way is for the app, e.g. terminal, to intercept and draw them directly, not through the font. Otherwise you’ll get horizontal gaps too https://github.com/tonsky/FiraCode/issues/449
That looks like a rendering bug in the application. There's no fundamental reason they should have either horizontal or vertical gaps, if character cells are rendered at the correct size.
font metrics are fractional at most sizes. You can optimize font for one particular size, but for any other, there will be gaps