The release of Firefox 4.0 may still be nearly a year away, but the excitement for the new version is already growing. In July, we revealed the first images of Firefox 4.0. Now one of the designers behind the browser has shared on his blog updated mock-ups of the new design.

While a lot of the original concepts from the July images are still intact (e.g. a combo stop/refresh/go button, color changes based on your actions), there are some new details revealed by designer and Mozilla contributor Stephen Horlander.

Here are the new mock-ups of Firefox Firefox 4.0, along with an explanation of what’s new in this iteration:

The New Design Changes

As we’ve previously noted, the Menu Bar has changed from version 3.5 to 4.0. The biggest change was that the stop, refresh and go buttons have been combined into one color-changing button. But in these new mock-ups, there is a new button: the App Button.

According to Stephen, the app button works a lot like the Windows 7 and Office 2010 menu button, which condenses a lot of those options into a single location. Instead of having File, Edit and all of those options take up space at the time, there would only be a single “Firefox” button at the top of the browser.

The look of this button, though, is still up for debate. In fact, there are multiple proposals for how it should look. See for yourself:

While the App Button may be the biggest change, it’s not the only one: the toolbar button is getting touched up to be more visible on multiple backgrounds, and the location bar is now recessed (rather than floating). Finally, the bookmarks toolbar is getting some new functionality — if it hasn’t been modified by the user, it will not appear by default.

Here are some more images for you to gawk at. Let us know what you think of the new changes in the comments:

Firefox 4.0: Even More Images

Firefox 4.0 with bookmarks toolbar:

Firefox 4.0 with tabs under the navigation bar:

Firefox 4.0 with bookmark bar between tabs and navigation bar:

Comparison of Firefox 4.0 and Firefox 3.5