After the rather tame Firefox 12 release, we'll be seeing more interesting stuff coming in Firefox 13.
New Tab Page - There's a new tab page showing your most frequent visits. Re-order, pin or even remove sites from the new tab page at will.
SPDY on by default - SPDY protocol is not enabled by default. Sites that use SPDY will be faster than ever.
Smooth scrolling - This is gonna take some getting use to. I'm so used to the old scrolling in Firefox it feels odd to have smooth scrolling.
New Home Page - A newer default home page that's more functional. Too bad Ubuntu replaces it with their own. Maybe Ubuntu 12.10 will have a similar redesigned home page?
Want to know more? It's all on the release notes.