Firefox has finally become a true multi-process browser, the one feature of chrome that I actually wished firefox had. In short, it's more stable, more secure, and faster at the cost of higher resource usage. It still doesn't use as much memory as chrome, but it's more than the older versions. I think the trade-off is worth it, you won't really feel the burn unless you have 2 GB or less of RAM.
They're also backporting the CSS engine from servo, which is properly multithreaded and thus much faster.
Unpopular opinion: I actually don't mind pulseaudio. It's much more feature-rich than straight-up alsa and is stable now that there are more devs working it than just Poettering.