Firefox is lagging behind on a lot of the more esoteric web APIs. I think it's a choice by the Firefox team, and a valid one given the effort required, Mozilla's relatively limited resources, and how few devs actually want or need these APIs. Firefox is more like a "website viewer" than a "web app host" like Chrome and Safari. I think that's what a lot of Firefox users want, and why they choose that browser. It might even be useful to differentiate between them instead of calling everything a 'web browser' in the future.

To that end I don't think it's a fair criticism of web app developers or Firefox that apps like this don't work in your browser.