We last looked at what Firefox 4 might look like in July. Since then, Mozilla has started devoting its design efforts to finding a seamless spot for an "application button", offering tab placement choices and fine-tuning the small things.
Mozilla contributor Stephen Horlander posts some "iterations" of the Windows design mockups for Firefox 4 on his personal blog, and seems to make a point of stating that Firefox 4, not 3.7, is where we'll see a real design change, despite the 3.7 designs we've already seen.
Key among the challenges to a 4.0 design are locating the Firefox application button, presumably holding all the action menus, and finding a comfortable fit for tabs and bookmark bars, however the users want them.
You can read about the other design choices being made, and see more screenshots and ideas, at Horlander's blog. Like what you're seeing? Left wanting more? Tell us all about it in the comments.