If the mockups in fact represent the next Firefox UI, we now have confirmation that Firefox is moving even closer toward Chrome, while adding a few pieces of its own flavor.
Besides cosmetic changes, the big news is the integration of the Home T, which is a permanent tab that cannot be removed by the user and will essentially become the Firefox homepage that holds all the information that is important to a user - access to emails, calendar data, news, social network updates, etc. The tabs themselves appear to be much more flushed out. As far as navigation buttons are concerned, the forward button is gone.
The Firefox menu receives a major update with a reduced menu that is based on icons and not on a list view anymore. The default configuration options are very limited, but can be extended by the user on demand. There is also a new full-size view of the window, which aims to improve Firefox' ability to work with web apps and uses Mozilla's One-Liner prospector add-on that forces the browser to share the location bar, tabs and menu items in just one line.
There was no information when this UI could be released, but Firefox 7 or 8 sounds like a reasonable estimate to us.