Add "tree" / "hierarchical" / "nesting" UI difference depending on the nesting of tabs

[tkr]

I believe that the recent enhancements, such as group tabs and vertical bars, are fantastic improvements! However, I feel that an important feature is still missing: a visual distinction for the nesting levels of tabs.

For instance, when I create a new tab, it represents a hierarchy level of 0. If I open a link from that tab in a new one, it would then have 1 ancestor, and if I do this again, it would have 2. This concept of "nesting" is what makes extensions like "Sideberry" (https://addons.mozilla.org/en-US/firefox/addon/sidebery/) and "Tree Style Tab" (https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/) so effective and user-friendly.

I suggest implementing an opt-in option that allows users to visualize the nesting levels of their tabs. This functionality could greatly enhance the user experience and organization of tabs.

[Jon]

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[ecl]

Yes please. Sometimes a set of tabs don't warrant their own group, but are all related in some way, maybe while still being in a group, so nice to still have a distinction that these tabs under this main/top tab are related. So an informal group or list within a group/folder of tabs.

[Alxndr_Drgn]

I would really like to see this implemented in Firefox's native vertical tabs.

For me, the most important part is not just grouping tabs, but preserving the actual parent → child relationship between them.

For example:

Search results
 → Article A
  → Documentation referenced from Article A
 → Article B

When I open a link in a new tab from an existing tab, Firefox already knows which tab it originated from. It would be extremely useful if native vertical tabs could use that relationship to automatically:

• display child tabs indented below their parent;

• preserve multiple nesting levels;

• allow a parent branch to be collapsed/expanded;

• optionally close an entire branch;

• allow tabs to be moved between branches.

Essentially, the core navigation model of Tree Style Tab, but integrated into Firefox's native vertical tabs.

Native vertical tabs are already very good, but without hierarchy a large number of tabs still becomes a flat list. Tree-style nesting would make the native implementation much more useful for research, documentation, development, and other workflows where one page naturally leads to several related pages.