Ah indeed, you seem to be correct about the lack of development. Just breezed through the popular topics, and the latest post from the dev seems to be about 4 years ago, and completed issues aren't even being closed. Though if you look on the Chrome Store page he replied to a comment in February. So apparently this addon is something like quasi-abandonware upon which he's still keen on collecting the (incidentally quite steep) registration price. And again, I wouldn't mind that at all, and would be happy paying twice the price, if it worked in a browser that allowed it to function properly.

Looking elsewhere, Tree Tabs is a good project with a motivated dev, that's in its infancy. Basically the only capability it has at the moment is to move tabs up and down and "in" and "out" of the hierarchy, which seems to be visually modeled on TO in an obvious way. Oh and looking now, it seems to have attained the ability to select and move multiple tabs (which has been "planned" in TO for 4 years), though with some bugs that seem to require a re-install in my browser. It was originally intended as a replacement for Tree Style Tabs on FF, whose future is somewhat questionable for several reasons (see my comment for links). As much as I have been devoted to TST, the readme on GitHub makes it clear that piro is very uninterested in expanding the capabilities of the addon.....which at the cutting edge here, would be potentially expending the very experience of web reading in fundamental ways. Am I the only person who thinks of things like 3d tab visualization webs of searches and related source pages to aid in research?

And as for open source, I doubt anyone would disagree with you (except one). In fact, someone started a port called FFTabsOutliner two years ago, but Vladyslav Volovyk asked them to remove everything related to this plugin, right down to the still-broken image example, and the project seems to be dead.

Sorry to be blunt, but asking $15 for a plugin updated 18 months ago and whose very few issues here have been ignored for years seems...outmoded, to be polite. The person programming Tree Tabs had made it to the basic functionality of this addon in like 2 months, and is open source, and is probably going to work in every browser, and he collaborates with his users astonishingly well. This makes me highly doubt the "this would be so hard to port to Vivaldi" excuse, as so many chrome extensions have done so in a very short time, with little staff.

To be still more blunt, no one will be paying for this in a few months, and I am saying this to encourage Mr. Volovyk to return here and start building on the promise it showed 4 years ago, and making it worth paying a frankly huge app fee over the three or four open-source and free versions available.

I have been an avid (perhaps even evangelical) user and promoter of vertical/hierarchical tabs for 15 years, and somehow missed this excellent plugin until just recently. I would be more than happy to buy it even if no extra features were added, but frankly the "stuck in chrome" issue is holding me back.

TO is an amazing (obviously one of the best) tools for serious web research available at present, but frankly Chrome is the worst browser for it to be used in. Aside from Chrome's memory hogging issues and ethical implications with Google policies like AMP coming out, the biggest problem is that Chrome does not and will not support a side panel, relegating TO to a kind of floating afterthought window that can be gingerly placed next to a small chrome window, at best. While just barely usable, it inevitably becomes awkward and feels very much like a separate program rather than an organic and excellent tab control, as it would feel like in a FF or Opera/Vivaldi side panel.

I think I'm going to hold off buying this until support is added for Vivaldi. The ease with which I've seen most plugins be ported suggests to me that it is probably a trivial matter, if not simply a matter of the "registration control" system that seems to want to lock it into my chrome sign-in name. Regarding this, perhaps a second addon could be created for Vivaldi to replace the chrome sign-in authentication? Any kind of workaround like this would be acceptable.

In Vivaldi this plugin would actually achieve its potential as an organic and integrated piece of the browser's navigation, and easily allow the kind of amazing navigational assistance and control is promises in the awkward Chrome interface.