I also noticed that not all rules from Fira Code are applied in Sublime Text. The working ligatures and the missing rules are both part of the Contextual Alternates (calt) feature.
Take for example this substitution rule:
sub \zero \x' @HexDigit by \x.multiply;
This is what it should look like:
The x is replaced by a multiplication x in hex numbers (and the colon is raised when it is placed between two uppercase letters, this is a different substitution rule which is not shown here). Both are not applied in Sublime Text.
The ligatures of Fira Code work though, and also work when I export the Fira Code fonts myself from the available Glyphs source file.