I interned at Figma the summer of 2016.

The graphics code of the core editor is written in C++, and compiled to JavaScript using asm.js. When WebAssembly becomes publicly available, it’ll compile to that. The backend is written in Ruby with Sinatra, using Postgres as the database, and is hosted on AWS. The front-end is written in TypeScript, using React, Redux, and CSS Modules.

This information will probably get out of date as the company evolves, but hope that gives you some context!