I feel like I should point out that Firefox won't be getting backported to Debian stable[1]. Until the next Debian stable, it seems that Firefox ESR 52 will be the only version of Firefox.
I downloaded Firefox from mozilla.org and configured a ~/.local/share/applications/firefox.desktop file so that it appears in the GNOME Shell menu. It auto-updates and works great.
Normally, yes. However, something on that page concerns me:
"Jessie and Stretch backports of Firefox release and beta are gone because of the requirement of rust to build them, which is not available in Jessie or Stretch. Please update your apt sources to use Firefox ESR instead."
I'm not sure how that Rust requirement and ESR 59 will play together; I'll assume they won't play together very well.
"won't" is a strong claim especially considered you didn't provide any source to back it. The page you've linked doesn't talk about such a possibility at all. This is only your wild guess right now.
[1] https://mozilla.debian.net/