How to include combining characters in subset?

I’d love to use Fira Sans for several projects I’m doing, but they are quite multilingual and require lots of characters made up of combining glyphs, and these don’t seem to work too well, even when _latin-ext_ is specified as subset.

The following is all on macOS 10.14.1.

**Safari** seems to handle it most elegantly: most glyphs load fine, even many that don’t exist as precomposed glyphs in the font (the diacritic itself is taken from a fallback font, but it is positioned on a Fira letter); only a few (like ȩ̃ and – for some strange reason – also a simple schwa, ə, which is definitely present in the font) seem to fail and use a fallback font:

**Firefox** manages glyphs that exist as precomposed glyphs fine, but chokes on combining characters. Instead of using a fallback font, the combining characters appear to be treated instead like modifier letters, hanging loosely in free air to the right of the letter they’re supposed to combine with. Bizarrely, even though the basic schwa, ə, reverts to a fallback font, schwa plus a combining character (like ə̄, schwa with macron) is treated like a ‘regular’ composed glyph and shows Fira’s schwa with a misplaced macron:

**Chrome** and **Opera** seem to act identically here: they choke on all compositions that don’t exist as precomposed glyphs (like Safari, they revert to a fallback font), including schwa (with or without combining characters):

For testing purposes, refer to [this JSBin](https://jsbin.com/juwihohafa/edit?html,output).

I’m guessing much of the weirdness (like Firefox suddenly recognising schwa if you add diacritics to it) is down to browser implementation and quirkiness, but one thing at least seems consistent: the combining characters do not appear to be available at all, though precomposed combinations are. Schwa must be getting sent along, but is somehow still not available.

Is it not possible to get the combining characters along with the font? Should they not be part of the Latin Extended subset?

Alternatively, is there a way of just getting the _full_ subset, with all 2,104 glyphs (assuming the version of Fira served through the Google CDN has the same number of glyphs as the downloadable one)?