Firefox is just less usable. It's slower and a lot of config options are missing. Stuff that is common in enterprises and has buttons in Chrome (e.g. "add the intranet search to your browser!") is hard in Firefox (you have to write and serve a specific XML file somewhere).

Also it has a big incredible redesign every 2 weeks, as far as I can tell from HN, while Chrome's UI is mostly boring and stable.