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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
Re https://github.com/piroor/treestyletab/issues/2494#issuecomment-599844898 flat tabs mode (my preference) is not tested well, and https://github.com/piroor/treestyletab/issues/2717 there'll never be a menu option to flatten, however as you might have realised, flatness does seem to be robust enough.
Sidebar Tabs features in the screen recording at https://github.com/FilipePS/Traduzir-paginas-web/issues/92#issue-812667545 (but please note, use of the extension there is coincidental – Sidebar Tabs is not contributory to the bug in the recording).
At one time, trees were forced. At my request, the developer made them optional.
I assume that the speed is partly a consequence of not integrating with the context menu items of other extensions and of Firefox. See for example the discussions at and around https://github.com/Drive4ik/simple-tab-groups/issues/749#issuecomment-783171538