This is a Firefox add-on which provides ability to operate tabs as "tree". New tabs opened from the current tab are automatically organized as "children" of the current. Such "branches" are easily folded (collapsed) by clicking on down-triangle shown in a "parent" tab, so you don't need to be suffered from too many visible tabs anymore. If you hope, you can restructure the tree via drag and drop.

Such a tree of tabs will behave like a visual browsing history for you.
- For example, if you see a list of search results for a topic, you'll open each search result link in new child tab.
- For more details you'll also open more descendant tabs from them.
- You'll easily dig and dig deeply, without lose your browsing trail - if you hope to go back to the original search result, you just have to switch to the "root" tab.
Moreover, you'll treat tree of tabs just as "grouped tabs" for similar topics.

Anyway this addon just provide uncolored "tree" feature. Please enjoy as you like! Tips:

"F1" key toggles the TST sidebar.
"Tree Style Tab" toolbar button also toggles the sidebar. If it's unnecessary, you can remove it from the toolbar.

Helper addons: There are some addons extending usability of TST's sidebar.

Multiple Tab Handler allows you to select multiple tabs with long-press on tabs. It also allows you to close mutiple tabs with long-press on the closebox on tabs.
TST Bookmarks Subpanel allows you to show a small "Bookmarks" sidebar panel below tabs in the TST's sidebar.
TST Active Tab in Collapsed Tree shows a small tab on a collapsed tree as an alias for the last active tab under the tree.
TST Lock Tree Collapsed allows you to lock arbitrary trees as collapsed. (This was a built-in feature on TST 3.3.0-3.3.6, and now separated.)
TST Tab Drag Handle provides a small tooltip on tab labels to start dragging of tabs for specific operations. (This was a built-in feature on TST 2.6.0-3.3.6, and now separated.)
TST-MiddleClick allows you to run "undo close tab" or "close currently active tab" command on middle click on the sidebar.
Tree Style Tab Mouse Wheel allows you to switch active tab by wheel scrolling.
Tab flip for Tree Style Tab allows you to move focus to the tab previously focused, by clicking on the active tab.
Tree Style Tab Focus Preceding Tab on Close focuses the previous tab instead of the next tab when a tab is closed.
Tab Unloader for Tree Style Tab allows you to unload tabs by clicking on them in the sidebar.
Move unloaded tabs for Tree Style Tab allows you to move tabs in the sidebar without them becoming active.
Tree Style Tab in Separate Window allows you to open the Tree Style Tab sidebar page in a new window.
Auto Tab Discard supports the fake context menu in the Tree Style Tab sidebar.
UnloadTabs supports the fake context menu in the Tree Style Tab sidebar.
Bookmark Tree for Tree Style Tab allows you to bookmark and restore trees.
TST Hoverswitch allows you to switch tabs by hovering over them.
TST Colored Tabs gives custom background color for tabs based on their domain.
Add Last Active Class To Tab helps you to give custom appearance for the "previously active tab".
TSTのタブを閉じるボタンの挙動を変更 (tst-change-close-tab-button-be) allows you to close the parent and its all descendants with a middle click on the closebox of a parent tab, whether the tree is expanded or collapsed.

Latest information is available at the repository.