Windows, tabs, bookmarks, notes, and more... all mixed together in one place, a highly structured living document that you and your browsers build together to organize your online life. It's like Tabs Outliner or Tree Style Tab, but better. Features: (there are a lot... maybe get a snack and drink before you dive in)

Nested vertical tabs and groups: Like the "vertical tabs" and "tab groups" function in some browsers, but way more powerful. Windows, tabs, bookmarks, and notes are all mixed together in one big outline or "tree" that you can organize however you want. Each item in the tree is a "node".
Handles many many tabs: Easily and reliably handles trees with tens of thousands of nodes. Whether you have 5 tabs or 50,000, this extension can handle it, even on old or slow computers.
Automatic backups: Never lose data again. Saves a snapshot of your entire session on a configurable schedule, in plain text files, so even if your entire browser config gets deleted, you won't lose your session.
Unload tabs to save memory: If you unload a tab, it stays in the tree and functions kind of like a bookmark, using no resources and no space in the tab bar. Then if you open an unloaded tab again later, it goes right back where it was. Tabs unloaded this way are fully unloaded, making them even more lightweight than the "discarded" tabs some other extensions use.
Save and restore entire windows: If you close a window, it stays in the tree as a saved window, complete with all its tabs, bookmarks, notes, etc. Then you can re-open that same window again later, with all the same tabs.
Tab groups: Tabs are grouped automatically into nested structures, and can easily be operated on as a group. Move groups around easily, even between windows. Can also mark multiple groups and do batch operations on all of them at once.
Open new tabs next to current: If you're in the middle of your tab bar and open a new tab, it won't open alllllllll the way over at the right edge, where it could be hundreds of tabs away. Instead it opens immediately to the right of the current tab, automatically grouped as a child of the current tab.
Notes and tasks: Take notes, both short and long notes, right in the tree. Organize your tabs into groups, nested as deep as you like. Add checkboxes to indicate tasks, using a variety of task states, and track progress using todo lists and automatic progress summaries.
Full keyboard and mouse support: A keyboard is strongly recommended for most tree management workflows, but a mouse is easier for new users, and has a few extras... like drag a branch out of the tree to paste it as Markdown, or drag text into the tree to add notes.
Cross-browser compatibility: Works in nearly all browsers, so you can use whatever browsers you like. For example, close your session in Chrome, then open it in Firefox right where you left off.
"Tree Style Tab" style sidepanel: Displays tabs in a side panel, showing only the tabs and notes in the current window. This is "Window" mode.
"Tabs Outliner" features: Can import your old sessions from Tabs Outliner, for a nearly effortless upgrade. Also, to help ease the transition, it can operate as a full session view in a separate window if desired.
Free and Open-Source: The only way to build software which lasts long-term.
Privacy: The extension is designed to make sure you own your data, and prevent any data from ever leaking to third parties without your consent.
Sync: Coming soon. Synchronize your tree in near real-time between all your browsers, even if they're not the same type of browser. Can use a self-hosted server for maximum privacy, with no data leaking to corporate clouds.

This extension is meant for people who need to keep track of many things. You can build your own entire workflows in the "tree", like a personal Kanban of sorts, or a "GTD" / "Getting Things Done" workflow, right in your browser.