Have you tried Tab Outliner? [1]
Compared to TST it has two (major?) drawbacks upfront:
1. It sits on a separate window next to your other browser windows.
2. Some of its features are paid features.
I always thought that TST for Firefox has ruined all other browsers for me but I actually like Tab Outliner more than TST. Here are my reasons:
1. It manages all my browser windows in a single tree view. Separate windows are children of the current session.
2. Its has a much better keyboard support like rearranging tabs with your arrow keys while holding CTRL or indent/unindent a tab further into the tree with the usual TAB / Shift + TAB shortcuts.
2.1. Even rearranging your tabs with the mouse seems much more deterministic than in TST. In TST I struggle with rearranging a tab as a new child vs. as a new sibling tab.
3. You can unload tabs or entire subtrees (collapse its children and then press the green unload button) to free your RAM.
4. Google Drive backup feature to sync your entire tree to your other computers. You can restore your entire tree or just a subtree on an other device just via drag and drop.
I only use a subset of all Tab Outliner features but it is already giving me a much saner tab management experience than TST could do. The better keyboard support and syncing my tabs across devices while preserving the tree structure are essential features that I can't have with TST on Firefox. Personally I have more sympathy with the TST developer (always nice on GitHub) but Tab Outliner is still the better browser extension.
[1] https://chrome.google.com/webstore/detail/tabs-outliner/eggk...