I use Firefox nearly every day and it's pretty good. If it was the only browser in existence it would be hard to complain. However, when switching between chrome and firefox I'm always much happier when I switch to chrome than I am when I switch to firefox. And it's not because of site rendering or anything like that, it's always a matter of UI, speed, and resource usage. The firefox UI is pretty clean but it has just too many weird clunky edges. For example, there are many situations where chrome just "does the right thing" while firefox does not (and it's more common for chrome to be easily configured to do something different if you want it to), this is especially true with handling tabs and the new tab page. Updates are another example though, chrome's updates are fast and nearly painless, firefox is a lot better than it used to be but updates are slow and it's far too common to launch the browser and for some reason see that it's not updated.
There are also many cases where doing "ordinary things" while browsing will lead to firefox slowing to a crawl or using crazy amounts of resources. Chrome is a notorious memory hog and yet somehow it manages to behave better than firefox in most situations, at least for my browsing habits. I also really like chrome's search integrations, if I want to search wikipedia all I need to do is hit ctrl-t then type "wiki search string" and boom I'm there. Same thing for youtube, amazon, etc. It's a small thing, but it's the addition of all the small things which make for an overall superior experience.
For reference Mosaic was my first browser and I've always been open to switching over to whatever browser had the best experience. I was using phoenix project builds before they renamed them, I was using Chrome back when it was in beta (speed, process isolation, minimalism, better acid2/3 scores, what wasn't to like?), and I keep checking on other browsers to see if they're worth switching to. The Mozilla org is great and firefox is a solid browser but it needs to up its game if it wants to be seen as a state of the art browser.