This extension provides the ability to work with tabs as "trees". New tabs opened from the current tab are automatically organized as "children" of the current tab. Such "branches" are easily folded (collapsed) by clicking on the arrow shown in the "parent" tab, so you no longer need to suffer from too many visible tabs. If you want, 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, each search result link will open in new child tab.
- New tabs opened from these "child" tabs will appear as descendants of their originating tabs.
- You'll easily dig and dig deeply, without losing your browsing trail - if you want to go back to the original search result, you just have to switch to the "root" tab.
Moreover, each tree will reflect a group of tabs on a similar topic.

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.

Permissions: This extension needs following permissions to work as expected:

Required:
- Access browser tabs: to list both active and non-active tabs in the UI.
- Access recently closed tabs (access to the session information): to save tree information of tabs (structure, collapsed/expanded state, etc.) to tabs' session information and ask to Firefox to manage them as bundled.
- Display notifications to you: to present information and navigate users without blocking.
Optional:
- Read and modify bookmarks: to simulate bookmark related commands in the tab context menu. / to detect tabs whether they were opened from bookmarks or not.
- Access your data for all websites (run scripts on webpages): to detect the Ctrl key is pressed/released and prevent needless expansion of collapsed trees while tab switching with Ctrl-Tab/Ctrl-Shift-Tab.
- Hide and show browser tabs: just for automated tests, not required on regular use.

Extensions extend TST: TST provides an API for other extensions. Some extend the behavior of TST's sidebar panel:

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 More Tree Commands provides more context menu and keyboard shortcut commands to manipulate TST's tree.
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 Active Tab on Scroll Bar shows a marker to indicate the position of the active tab, on the scrollbar.
TST Auto Group Tabs provides ability to group newly opened tabs automatically in various conditions.
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 Indent Line provides indent lines like Visual Studio Code's one.
TST Open Bookmarks as Partial Tree allows you to open only some bookmarks in a folder as a partial tree. Moreover, it also provides ability to open tree of container tabs from bookmarks.
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. (A similar feature is now built-in in TST since v3.2.0)
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.
TST Select Random Tab adds a context menu option for selecting a random tab in a Tree Style Tab tree.
TST Tab Search provides a search field (as a subpanel) to filter opened tabs in the sidebar.
TST Fade Old Tabs sorts all tabs in 3 categories – 'recent', 'older' and 'oldest' by latest visit time, and applies different opacity to them.
TST Colorize Tabs allows to set individual color for each tab in order to highlight it.
TST New Tabs First puts a newly opened root level at the top of the sidebar.

Latest information is available at the repository.