Tree Tabs Refreshed от BapoHka

Tree Tabs Refreshed puts your tabs into a real tree in the Firefox sidebar, with
groups, folders and full session management.

Tree
Real parent/child hierarchy with expand/collapse, configurable maximum depth and
drag & drop reordering.

Groups and folders
Split tabs into named groups with an optional groups toolbar; nest tabs into
folders with custom emoji icons and colours. Optionally show only the active
group's tabs as native Firefox tabs.

Sessions
Save, import and export whole sessions or single groups. Automatic rotating
backups run in the background, so the tree survives an unexpected exit.

Everything else
Pinned tabs bar, search by title or URL, per-tab and per-folder colours,
theming, regex rules that sort matching tabs into a group, and configurable
mouse actions.
Interface in English, Ukrainian, russian, and Swedish.

This is an unofficial, maintained fork of Tree Tabs by kroppy. It focuses on
crash-safety, memory use with large tab counts, and bug fixes.
Source: https://gitlab.com/bapohka/tree-tabs-refreshed