Try installing ttf-fira-code instead of ttf-firacode-nerd. According to pacman’s package descriptions:
$ pacman -Ss ttf-fira-code
community/ttf-fira-code 6.2-2
Monospaced font with programming ligatures
$ pacman -Ss ttf-firacode-nerd
community/ttf-firacode-nerd 2.2.2-2
Patched font Fira (Fura) Code from nerd-fonts library
Now I could be wrong about this, but is it possible that the fonts in the ttf-firacode-nerd package doesn’t support ligatures? Only ttf-fira-code’s description said it has ligatures.
I also checked the ryanoasis’s nerd fonts github page.
The version number for the fonts seem to match with the ttf-fira-code package (version 6) but not the ttf-firacode-nerd package.