These days FF is just as fast and lighter than most Chromium browsers. With a better curated add-on store (my god google store sucks), a clean UI (which they'll hopefully stop messing around with), and more importantly, it just works for day to day use, and is customizable enough for most people's needs.

While Mozilla have themselves to blame for a lot of the decline. A big part of it is Google's dominance over the web and android, pushing Chrome on everyone's face for years. Bundling Chrome with software, annoying pop-ups on android telling you to use Chrome, pre-installed on android, pushing it on their main page... There's so much of it. Of course there's also Apple's walled garden preventing it from going there, as well as years of MS doing what Google does now.