Almost no new features, other than some flexilibility with the new Firefox menu (you can add extension buttons to it, for instance).
Lots of customisation options have been removed (with the justification that it prevents inexperienced users from 'breaking' the browser), like the navigation buttons being locked to the address bar, the inability ability to move the refresh button (urgh) and the addon bar. Most can be restored though the Classic Theme Restorer addon, though it broke another of my addons last time I tried it.
True, the new Firefox menu is much improved, but there still seems to be no way to access it via the keyboard so it will probably remained largely unused by me.
I like the new placement of the real menu bar (when enabled), much better than in the previous versions.
I actually think that the orange 'Firefox' button was an important part of the browser's branding. Sure, it was blatantly stolen from Opera, but it meant you instantly could tell which browser someone was using. I think moving that functionality to the new menu button was a mistake, and it makes the browser look a lot more generic/Chrome-like (and is also against all existing windows conventions which says administrative functions should be in the top left).
But, in terms of day-to-day usage, its location doesn't really matter much.
actually that one button appeared in firefox designs much before opera - there was a big drama about it back then.
opera had the same design but released much faster than mozilla did.
I like Firefox and will continue to use it, but I just don't see what good this update is supposed to accomplish.