I remember Mozilla "evangelist" employees hard at work, tweeting "version doesn't matter, it's the changelog that matters, chrome team could have changed two background colors and did +1 to the version counter".
Great battle move. Then, Firefox 4 got released and everyone hated the new interface. Even ex-mozilla cofounder JWZ complained that it feels beta even though it's a fully mature product out for a decade now and that's unacceptable. Also, No mp4 support. Oh yeah. They added it in Firefox 20 finally (then no webm support. They finally added it in January 2019. Good job)
In the meantime, Mozilla engineering kept fooling around with projects that went nowhere. Fennec Fox for mobile, what happened to that? THE JAVASCRIPT PHONE! FLASH TO JS CONVERTER, where is that now? That JS only Video Codec ORBX.js? that Eich said "I saw the future today" 10 years ago, and it's still not out yet. PDF JS (in it's first release made opening large PDF files on Firefox impossible) etc etc. Many good things came of from it too, like asmjs (ancestor of wasm), but in general there seemed a direction of everywhere, neither arriving here nor there.
Mozilla then switched the default search engine of Firefox to Yahoo! At a time when Yahoo was in the spotlight for rapidly dying and looking for a pity-acquisition!! It also came bundled with great bugs that kept resetting the default search engine back to yahoo - https://support.mozilla.org/en-US/questions/1206101
To give praise where praise is worth, Mozilla did realize their mistakes got back from their 9 year vacation (2009-2018) and Firefox today is finally very stable and relatively fast compared to Edge and Safari.
I know I am sounding trollish and bitter, but the truth is that I am just sad at how Firefox fell. What happened? Did people get lazy as their savings grew? Where key individuals poached away by Google and Apple? Did they burn out? Something surely must have happened...