Firefox (on Mac) lacks three things which I hope they fix them ASAP:
1. Compared to Chrome, FF just makes the Mac insanely hot. Sometimes if a website is stuck at loading, the CPU usage goes seriously high, resulting in temperatures around 80˚ Celsius, and MBP fan rounds at ~5000 rpm.
2. FF doesn't support page scrolling with keyboard shortcuts like Chrome does. On a Mac, you can [big] scroll up/down using alt + arrow keys. "Home" and "End" could also be achieved by Command + arrow keys. On FF however, I haven't found the same shortcuts.
Update: It has a weak form of this feature (but still requires your MBP to have "Home" and "End" buttons on it!): https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perf...
3. FF used to be faster than Chrome at startup, but now on FF 57-58, when you type in the omni-bar, it takes quite a long time for FF to figure out where it has seen this before. In other words, it can't quickly tab-complete what you're typing among bookmarks, history, etc.
I switched from Chrome to FF a while ago, mostly because of the Quantum and the new slick modern UI. But due to these annoying problems, I switched back to Chrome.