There are two things that I find limiting. First, the configurability has been seriously reduced. I really dislike Firefox's UI, and it's no longer possible to make the browser look and feel as I prefer. I can get most of the way there -- but to even do that much is a rather serious pain in the butt. Second, the neutering of the extensions hurt badly, and the only way I can retain the functionality that I want is to use something pre-Quantum.

Waterfox has neither of these problems (well, the extension problem is starting to impact Waterfox, which is why I'm no longer updating it. I need to find another browser, but I haven't found a different one that works for me yet. Until I do, Waterfox it is.)