Flaming hot take that's probably gonna get me downvoted to hell: releasing a new version of Firefox on mobile before there was a high level of extension compatibility killed a lot of the momentum it had going for it.
I know that maintaining two separate codebases is a huge challenge, but having the power of all desktop extensions on mobile was a pretty big draw and being told that only a small fraction of them work probably turned off a lot of people. I kinda wish they just put the old codebase in maintenance mode with bug fixes since it was pretty feature complete while rolling out Fenix as a beta.
Sandboxing also still sucks even after the rewrite, (still no Fission!) which is unacceptable in today's day and age. I say all this as a Firefox diehard on all platforms, but I'm tired of people acting like it's a magic bullet to kill Chrome. It still has a lot of problems, including its resource usage. I'm hoping Servo takes off soon, because that's what made Firefox so fast on desktop even though only a fraction of it was implemented.
/rejerk So glad my furry porn website userscripts work again!