Nothing really drastic on the outside, just lots of security features under the hood. But who cares, it’s a new versionm and newer is always better. 🙂 Check the changelog for more info.
EDIT: Looks like HTML5 video is now a prefered video rendering method, at least on Youtube. Even though I have Adobe Flash installed, it’s using HTML5 mode. Youtube also says more HTML5 video standards are supported than with Firefox 36. So I guess this is one of the largest changes even though it’s not listed in changelog for some reason.
You can already grab Firefox 37 through their FTP: