Firefox still feels a tad slower than Chrome, but it's normally fast enough.
The big problem I have with Firefox is developing extensions. Developing browser extensions in Chrome vs building them in Firefox is like night and day. While I'm not an expert, I've built a few extensions for each browser.
Building one in Chrome has been a very pleasant experience for me - the documentation is good and up to date and things work very logically.
But developing Firefox extensions is an exercise in complete frustration. Documentation is hard to find and out of date. Functionality that is needed is frequently changed and deprecated. Common things are made difficult and frustrating. There are code examples found all over the web, but most of them are out of date.
Purely speaking with my developer hat, I would not build another Firefox extension if not absolutely required to for a specific project. And I dislike a lot of Google's policies and love Mozilla's nature, but I think Google is clearly much further ahead in terms of offering a pleasant development experience.
I don't know what the answer is, but for me personally Firefox isn't working.