Yeah it lags behind because Chromium is developed by Google, which is the 4th biggest company in the world. And Safari is obviously from Apple which is the largest company in the world. I don't think the fact that Mozilla lags behind should upset anyone. The fact they can compete at all is impressive I think.
Unpopular opinion, brace yourselves.
As a web developer, I would love to root for Firefox but they've made some really odd decisions regarding the implementation of web standards (which are published on the Mozilla MDN site, oddly enough), async/defer script loading order for example. Firefox is also often multiple years late with implementing new tech, being surpassed by Chromium and even Safari most of the time.
While I love the non-profit style of Mozilla and think competition in the browser space is a good thing. The reality is just that their browser lags behind the other two. Firefox is a large part of the reason polyfills are still used in this world of evergreen browsers, and requires multi-browser testing/tweaking even though I exclusively follow the standards written on the MDN website...
save