I'm here to put some salt over the topic.
Those who already talked with me know I'm a heavy Arc user.
Arc has this super convenient "Tab Folders" feature and this is the second most important feature that prevents me from moving to Orion as a default Browser.
First, what I don't want from what I read just above:
I don't want tabs to nest other tabs in the tree view.
I don't want to rename tabs to make them appears as folders but still be tabs.
I don't want to click on a folder and the browser opens every links in the folder.
What I want for this feature is this:
I want folders to be "real" folders. When you click on them -> You open/close them.
When a folder is open in tree view, you can click on the tabs in it to open them.
These tabs have to be closed until you open them and they have to offer a way to close them without removing them from the folder, allowing them to persist without being loaded.
We should be able to rename folders.
We should be able to nest folders in folders.
Browsing from a tab (placed in a folder) should either create a new tab and then we can consider tab-in-folders like pinned tabs but, not compact and in folders OR/AND provide a way to reset the state of the tab-in-folder to the original URL when we started to browse. Anyway, the original URL should persist for this tab after the browser is closed and reopen and the state of browsing has to be forgotten through tab close and browser restart.
Horizontal tabs don't fit the concept of folders. But, Google Chrome and Beam are example of Tab Group (folder) that are useable with horizontal tabs. However, I think all the power of folders can't be exploited in Horizontal Tabs, as the tree view, the best way to actually use folders, is not even a concept for Horizontal Tabs.
Tab Folders are similar to Bookmarks but are actually part of the browsing. And so, tabs open outside any folders have to be completely distinct. These tabs should be draggable over a folder to put them in.