I've been using this for personal use for around 2 years and I figured it was time to share it with the world. Criticism, issues and PRs are welcome. Thanks!
It's good that you made an alternative but it's not serious. I need to run Firefox, enable JS, reload the page, there is no "top menu" or install button, the open/save button does nothing, I can't choose my own fonts, and I guess it doesn't work if I don't have an internet connection.
I do most of my work in browser tabs. It's much more convenient for me to have a plaintext editor app as a tab I can position among my other tabs, rather than another window.
My email is a tab. My word processor is a tab. My files in the cloud are a tab. It makes sense that my plaintext editor should be a tab too. It's just UX consistency and convenience.