Hah, I always thought M+ 1m was my little secret that I had discovered; apparently it's the fifth most popular coding font. It's significantly more narrow than most monospaced fonts while still being easy to read, allowing for more side-by-side editor windows.
I really like the way Fira Sans looks, and I used it for a while, but M+ 1m has a huge practical benefit. And it still looks great, too.