The Firefox add-on Tree Style Tab allows hierarchical tab ordering.
Does any other main browser have such a feature or provide an extension for it?
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Google Chrome has a similar extension: Sidewise Tree Style Tabs. The functionality is similar, but it behaves a bit differently...
Because of the way Chrome handles its GUI, extensions don't have as much freedom to take space inside the browser window. So that means that this extension actually opens in a small separate window.
In Opera, there's something similar available, but simpler. It allows stacking tabs, but no tree structures (e.g stacks inside stacks) are allowed.
Opera now has Tree Tabs which is quite similar: it is displayed in the sidebar etc.