You may need to specify the XFT spacing explicitly.
The font may not be declared mono-spaced,
the solution is to force this through the font specifier.
Fira Code-13:spacing=90 (dual spacing)
Fira Code-13:spacing=100 (mono spacing)
I think you misunderstand what's been included in Emacs 27. Emacs 27 includes a new font backend based on the HarfBuzz library. The front end bits still need to be worked on according to Eli Zaretskii in this reddit post
I'm wondering if anyone is working on native support
It's already supported in Emacs 27, but someone should code the Lisp ...
I've found a working solution with the code below:
in my case, scaling didn't work because my variable-pitch configuration specified the :height in the format 140. i switched it to the format 1.3. and now it works.
(thanks Basil for the pointers. emacs -Q will come in handy again i'm sure.)