So, perhaps some of you know firefox exstension called tree-style tabs https://addons.mozilla.org/PL/firefox/a ... style-tab/

This approach has some interesting benefits over horizontal bar, namely:
1) Easy folding and unfolding of the opened tabs into groups.
2) Opened tabs are organaized in a meaningfull way.
3) When large number of tabs is open you can easily scroll the tree, fold unused groups, etc.

I think that tree style tabs could be adopted into some of the kde applications while preserving those benefits. Obviously, rekonq web browser rises to the top of this list, but any other application that typically hold a large number of the tabs, usually opened as a resoult of action taken in one of already existing tabs should work well with tree tabs, although horizontal tab panel indeed takes fairly large ammount of space.

That would be a very good idea. It will enhance navigating among the open Windows.

An even better design is Sidewise Treestyle Tabs.

It is a better conceived implementation for Chrome, but isn't as reliable as Tree Style Tabs, but it is hard to tell whether it is due to Chrome's crash prone nature.

In my opinion even better can be achieved. Take current Folder-dock of Dolphin and add following:

• Show items of folders in addition to subfolders.
• Folders have handles to open and close tree-branch like currently. But clicking folder opens it in the widget in stead of the content area.
• Add button to return to tree-view.

This abilty to either show only one folder or full tree enables to separate working in one group/task and inter group management.

Where I would want it: web browser, Kate, media player, picture viewer. Anything where you have bunch of items in a tree-like-structure and want to view the contents of the individual item in the main area.

PS: For example I have 741 tabs in 60 groups in 4 levels in Firefox. Because of instability and bugs i cannot use tree-style tabs. Instead I use just Tab Group Menu and some addition addons like Unload tab.