Thunderbird 15 is coming out this Tuesday, August 21st and there are a couple User Interface (UI) changes as part of the new Australis UI. The tabs have been redesigned to look like well tabs (similar to what you see in Google Chrome). There has been some other small refinements in appearance of the UI as well. Also, when you are customizing your toolbars, there is no longer an option for ‘small icons’. Note: The ‘small icons’ change currently only affects Windows builds. Linux builds still had the ‘small icons’ option (have not heard about Mac users). Interestingly though, looking on my Windows 7 laptop with Thunderbird 14, I don’t see much if any difference in size between the ‘small’ and normal icons.
Other changes in Thunderbird 15 include the addition of the ‘Do Not Track’ feature which has been in Firefox for about a year now. This will be found under the ‘Web Content’ tab in the security section of Thunderbird’s options (Tools > Options… > Security > Web Content). Not really understanding how this suppose to work with email though. Also the global search feature has been extended to include the ‘chat’ module.
More information about these changes will be available in the release notes once Thunderbird 15 is released. Note: these changes are already in effect for the final Beta build of Thunderbird 15.