My take on FF "decline" is that FF user share (%) dropped because the pie started expanding (especially on mobile). New internet users are likely to be Safari or Chrome users for whom the browser they have is good enough. And Firefox on mobile is simply not as fast as alternatives (the bar set by Chromium Android is very high), kinda painful to use unless you have a highend device, esp. JS heavy sites like Twitter work much better on Chromium.

This doesn't fully describe situation, but over last 3 years, mobile users went from 50% to 75% users in my prev company's data. And Firefox is nowhere in those mobile stats.