It doesn't work AT ALL as it claims, which is ironic given that it makes very few actual claims to what it's supposed to do. The author repeatedly refers people to his website from the add-on page, the welcome/updates window, the add-on options page, yet NO webpage exists! 404 Error with every single link, and the homepage is no help, and neither is the "/alltabshelper/" directory root!

This thing claims to move tabs, it technically CAN do this, but you need to do a weird clunky ctrl+click to select the rows, and then right click and select "Move Here" to move the tabs, there is no drag & drop. You might as well just grab and slide the tabs at that point (TIP: Other browsers let you ctrl+click the tabs directly for moves like this)!
[EDIT: You CAN Drag & Drop if you do the clunky ctrl+click first on the row, as if you're about to select multiple rows even if you only plan to move a single tab.]
Contrary to what you might expect you CANNOT move tabs between windows, which is where the majority of tab moving usefulness would have been for me. There also doesn't seem to be any way to handle "loading" of new tabs, which is the main reason I installed this. I wanted to be able to take my exported tab URLs and load them back up when desired. Along these lines, where it says this add-on handles "unloading" of tabs, what it actually seems to mean is it offers the ability to suspend tabs, which is a fair bit more tricky to manage than simply saving the tab info and actually unloading/closing the tabs. Also the Search function seems to be sort of messed up in a strange way. It works if you have the "all" view selected, but if you click the "search" view it gets really weird. I have a tab open in the same window in which I'm attempting to search, the tab is titled "Dynamic Periodic Table" yet when I go to type my search criteria (which incidentally says beneath the textbox "Please enter 3 or more characters"), I type the first TWO (2) characters "Dy" and I can see the tab I'm looking for pop up into the search results, yet when I keep typing to reach the 3 character minimum ("Dyn") ALL search results disappear completely. Finishing off the first word, or the entire title doesn't help. There also doesn't seem to be any way to pull the tabs history. Also since i'm mid-review, it dawned on me I hadn't recalled seeing any duplicates listed, even when there should have been, so I went back and tested. Apparently there's a strange bug with updated the list of tabs in the sidebar. I had created a New Tab, then copy/pasted the same URL of another tab, the page loaded, yet it wasn't listed in the "dups" tab of the Sidebar, perhaps it needs to be reloaded to pick up new tabs? That's not good design at all (Note: It DOES show up in the Button -> pop-up "All Tabs Helper" window). In the sidebar it seems to sit there thinking it's still just a "New Tab", which accidentally caused another issue, prior to realizing this I closed (from the sidebar) what I perceived to be an unused "New Tab" tab, yet in actuality it was a fully loaded & rendered webpage. Misreporting tab names can lead to SERIOUS issues!

So of the short list of SEVEN features it lists: Moving, Loading, Unloading, Search, View Tab History, Find Duplicates, & Closing Tabs; All SEVEN either don't work at all, or have some serious implementation issues that need to be worked out before people should feel comfortable installing this add-on (OK, only SIX if "Suspending" is equivalent to "Unloading" for your Use Case(s)).

The Good News is this add-on can easily redeem itself. It just needs to first fix the website so the documentation can be accessed again, and it should further define exactly what it does on the actual Add-On page. I assume I would have been better prepared for the current state of the add-on had the site been up. Also, it REALLY should have the ability to move tabs between windows, as well as check for duplicates & be able to search through ALL TABS in ALL WINDOWS... If it's still possible to do so, I know they've been cutting off Extension and Add-On dev's abilities to access certain APIs for security and privacy reasons. However, one thing that CAN be fixed still is implementing better click to drag & drop support, you should be able to go in, click and hold an item, and drag it down/up. Also, if the Sidebar is causing so many issues, you may be better off sticking with just the pop-up for newer browsers, at least then you'll know the tabs will always be up to date, which fixes the dups/close issues.

I'll be sure to keep an eye out for any fixes & updates posted for this add-on, or feel free to author respond to this review, and I will make sure to upgrade the review rating to what I know this add-on will eventually deserve.
// Running Win 7 Ultimate, with Firefox Quantum v.61.0.2 (64-bit)

PS- Since the website isn't available for suggestions, I'll mention here. I arrived at this add-on while looking for a single add-on solution that could *BOTH* [1] Export a list of URLs (optionally with Titles) for ALL tabs in either the current Window or ALL Windows (Export could just mean listing in a TextArea box), and ALSO [2] Take a List of URLs and Open all the URLs in various tabs (optionally with Lazy Load to only ACTUALLY load the site once the tab has focus). There are separate add-ons that do one or the other, but have not been able to find one that does both strangely. I think this feature would fit in well with All Tabs Helper.