This process works by getting access to devtools stylesheets using custom themes; also works in Opera with the Download Chrome Extension installed

1. install the Fira Code font on your machine
2. follow steps to install a custom devtools theme, e.g. zerodarkmatrix
3. find the appropriate stylesheet stable.css or canary.css in your browser's Extensions directory on your file system
4. edit the section CodeMirror-code to include font-family:"Fira Code"

Voila