Hi, I wouldn't say there's anything wrong with TextEdit. It's what I had used for many years. It does have all kinds of extra features like input suggestions, auto capitalization and auto-correct (I couldn't find a way to disable these in the app, maybe somewhere in the general macOS settings).
Anyway, it's not about which app is better. If you love the Notepad experience, then you know - it's something about its simplicity, and perhaps its familiarity as well.
About being serious - this is indeed a side project. Sorry about FF support - since the relevant FS API isn't available there I added an alert indicating that on page load.
It does work without an internet connection though (PWAs are great)!