ZenDivider by Barn

ZenDivider Extension creates a simple renamable divider so that you can divide and organize your tabs. It's ideal for vertical tab users.

Think of your tab list as a book; each tabs are pages in that book, and these dividers that ZenDivider creates are like a piece of paper that you write and place between those pages to help categorize them.

Create it, Move it, Rename it.
Create it, move it around to your desired position, and rename them by typing into the large & highly visible page.
If your browser is set to restore tabs from last session, the dividers remembers their names & are restored.

Many ways to create dividers:
• Toolbar Button : Click on the extension to create a new divider. Easier access by pinning the extension.
• Context Menu : Right click on any tabs to create a divider positioned before that tab.
• Keyboard Shortcut : There's no default keyboard shortcut, but you can easily set your own shortcut in browser's settings. See "Manage extension shortcuts in Firefox".
• Omnibox : Type 'zd' into the browser's address bar, type the desired name of the divider, then press Enter to create a pre-filled divider, or create an empty divider by typing 'zd' and press Enter immediately without typing anything.
(Zen currently has a bug preventing this from working, but it works as expected in Firefox during testing.)

Misc info
This extension does not connect to the internet or communicate with anything remote. It's completely offline and does not collect data.
This extension does not use any fancy frameworks; it's just simple JS & barebones HTML, so it shouldn't be heavy on resources.