Please don't disable extension, it must be enabled to retain trees. Sidebar can be closed, but extension must be enabled.
With new Firefox API, tabIds are lost after browser restart. Extension must keep track of all tabs positions all the time. This is unfortunately a limitation to all chromium based browsers and Firefox joins that trend.
Hiding tab bar is unavailable from Web extensions for now. Please join conversation at bugzilla to convince Web Extensions team to give us this possibility, here:
For now search for an alternative extension that just hides tabbar, for example Hide Tabbar (in this particular one, uncheck reopen tabbar when opening a new tab in options, it does the trick)
theming is another limitation, and will be for all new Firefox extensions. As new sidebar extensions are just web pages with special privileges to have access to firefox API's like tabs favicons, titles, we can activate, move them in tabs strip at the top and so on. Tabs in my extension are not real tabs with changed look, this is just a web page with generated buttons that link events with the browser. All new extension will work this way in the future. I did my best to give it at least color customization. And remember this extension is cross platform, it works in Opera/Vivaldi and now in Firefox, so I can't make colors match only one of the bunch.
Anyway you can download dark theme at (and hopefully you can share yours as well):
Tabs hierarchy management will improve with time as I implement more and more features, and not only me, it depends on Firefox team as well, as for example Firefox does not have "openerTabId", so new tabs opened from middle click will not append to the tree, you must set "Append orphan tabs" to "treat as active's tab child"
For this matter, you can also join conversation at:
Thank you for testing my extension in this early stage :)