Install this package via the Settings Pane or by using the Atom Package Manager:
apm install language-fift
The first version (0.0.1) factually a fork from atom-language-forth without any real fixes. I am inviting everybody, who involved into TON blockchain development, to make some fixes in sintax highlighting to make development of smart contracts in Atom much better!
Good catch. Let us know what about this package looks wrong to you, and we'll investigate right away.