It's still slower than other platforms but it's much faster than it used to be. I've been using Firefox exclusively on OS X / macOS for a year. The only problem I've seen is that it sometimes uses more CPU than is reasonable, which often appears to be due to Gmail's terrible implementation.

Since they made the big changes and become faster, I found Firefox faster that Chrome, except for Google pages.

Chrome is notoriously power inefficient, though, relative to Safari at least. One would think developers would notice.

It's hard to notice power efficiency when you always have your laptop plugged in.