https://addons.mozilla.org/en-US/firefox/addon/vertical-tabs/reviews/ Recent reviews for that extension mention "issues" with it. And one review indicates the developer of that extension has quit working on it.

Another reviewer mentioned this extension as an alternative. https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/

I am trying the Tree-style tabs and it looks like it will be great. However, there are no directions that make any sense to me. I have 2 questions. 1) When the tree first popped up, all my pinned tabs were grouped together in a box at the top of the tree which I liked. However, the screen had too much of a gap between the tree and the web page. I somehow figured how to get it looking right, but in the process all the pinned tabs are now in a vertical column taking too much space. How do I get them back into the box? 2) Is there a way to get the existing tabs I have into the tree style with child tabs under a parent tab? Or do I just have to start over or leave the existing ones alone and allow new tabs to organize? Edit: Problem #1 solved itself. I would still like to know the answer to problem 2 about my current long list of tabs.

Sorry, I don't use that extension. See if the developer's website has an answer for that. http://piro.sakura.ne.jp/xul/_treestyletab.html.en