From my experience it isn't very good. All I have seen it do is basically save a window session without much indication to be restored for later. I wouId suggest Onetab, Tabmerger or simple tab groups instead.

UPDATE Sep 22, 2022

BUG: Lost ALL MY window labels.

Started to label my windows. Did about 20-30. Then a short while after, lost all the labels (a week or a few days?)

Without the window labels become far less useful as a tool.

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Brilliant solution!

UPDATE Sep 12, 2022

2 BUGS:
- If search tabs, will only show the first 80 windows (I have 92 ATM with over 2100 tabs)
- when remove search, you can access all windows BUT the scrollbar disappears beyond the bottom of the window.

MORE Recommendations:
- when first open, bring CURRENT tab into focus (Tabs Helper addon can do this)
- when search, bring first resulting tab into focus
- default Window name to current tab of window. To differentiate from manually defined window name, make it italics or something
- option to sort named windows alphabetically
- add (x) to quickly clear search field
- STILL need speed imporovements!
(Tabs Helper addon can do much of this)

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From Jul 11, 2021:

Things I really like:
- Preview feature - saves SO MUCH TIME!!! (I have HUNDREDS of tabs open... currently 1600+)

- filters out all tabs that do not match, across ALL WINDOWS!

Things I'd like to see:
- ability to choose to NOT show windows that have ZERO matching tabs

- speed improvement... not sure why speed is inconsistent... sometimes it is fast, other times it is slow, even searching for the same thing

- ability to remember the last few searches, like is a small drop-down menu

To fix:
- if start typing too soon, misses some of the characters

- settings - changing size greater than 760 x 600 creates vertical and horizontal scrollbars for the list but does not create a bigger pop-up.... Please allow pop-up window itself to be bigger...

A-maz-ing!! This is hands down the BEST add-on EVER! I'm one of those, 87 tabs open in 3 different windows, persons. Tabby has SAVED me. It's slightly unnerving at first when it's hard to tell if they're "really" saved, but they are! I recommend you clean up your tabs as much as possible from the drop-down before you save them in Tabby so you don't wind up with too much junk in "Other Bookmarks" which is where your Tabby bookmarks will go. Thank you, TabbyBill!!

Very easy to use. I created a second window and renamed it. Closed both windows, the newly created one first. Then I reopened Firefox, but while thesecond window as availiable to open, the name I'd given it had gone.

Unable to drag tab from one window to another window.

Perfect. I switched off the preview and also made the window smaller and the changed the scale. All worked perfectly. Thanks a lot. Great work

Really good and does what it says. I do find scrolling through tabs to be very haphazard though and the shortcut for the search bar just goes to current tab. Great otherwise thanks !

It cannot do that much. There are better extensions for the same functionality and more.

That could be an advantage if you're looking for a minimalstic tool.

Currently with Ff 82 Tabby is not saving windows. Its still useful when searching a lot of tabs in different windows.

I like what this add-on can do but it is EXTREMELY laggy and unresponsive. I even turned off tab previews and tab details which helped a bit but it's still extremely laggy. I'm running on a MacBook Pro 2019 model with an 8 core CPU, and when I open this extension's popup window, it nearly pegs an entire core. I'm usually around 5-6% average usage on my CPU cores during normal browsing, but when I open the popup it shoots up to 15-25% average. You do the math. To be fair, I have 8 windows open across 2 Spaces, but each window has less than 12 tabs, and honestly if you're using a manager extension like this you've probably got at least half as many windows and tabs as me. I hope the developer works on improving the performance of this extension. It's very nice and only hindered by the performance. I will still use it.

edit: thank you developer for the response. I look forward to any and all performance improvements!