This is a Firefox add-on which provides the ability to operate tabs as "tree"
Free • Open Source
What is Tree Style Tab?
This provides tree-style tab bar, like a folder tree of File Explorer . New tabs opened from links (or etc.) are automatically attached to the parent tab. If you often use many many tabs, it will help your web browsing because you can understand relations of tabs. Latest information is available at the https://github.com/piroor/treestyletab.