I am using Firefox 66.0.3, addon version 3.4.3, Windows 7 64bit.
I forgot to mention that it happens not every time.
You can contact me at alexjzx (@) gmail.com.
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Add-on is really cool and it does the job well, but:
1) It takes too much time to load tabs, about 2-3 mins for ~60 tabs
2) Until it is loaded, you re not gonna to do anything to your tabs, because it will load tabs state in 3 mins and will erase all your progress(e.g. newly opened tabs)
3) Thus, it slows down starting your work with the browser.
For the rest, this addon is great and highly usable.

Absolutely fantastic add-on! Great for organizing many open tabs and windows. Note one thing that was not very clear early on for me is that each group is associated with only one window. So you cannot have multiple windows associated with a single group. Although I did not expect this at first, this approach gives a very fluid user experience. I cannot recommend this extension enough if you are like me and tend to keep many tabs and windows open for an absurd amount of time and return to them later.

One slight annoyance is how, from what I can tell, there is no easy way to convert a window that is not associated with a group to be a new group. If you select ALL open tabs in a window, use "Move tab to group" to a new group, it closes the window you were just on, changes the current window to another one, and changes that window to the newly created group. Expected behavior is to keep the window open when creating a new group for all tabs on the the current window.

The previous bug is fixed and I gave the extension 5 stars. But, it is very annoying when the extension spontaneously hides tabs when switching groups. Can you tell me how to disable hiding tabs?

I've been holding off updating Firefox until this add on became stable. Over the last week I have been running it in in parallel with my production environment with ~ 150 open tabs in 5 groups and had no issues with load times, overall performance or stability.
Consequently, I have today fully transitioned to FF V65.01 and Simple Tab Groups.
Am very happy. Great work! Thank you.

非常好用，在以前FireFox內建的「分頁群組」被拿掉之後，這個tab管理套件算是很不錯的。

"HAAAaaaaaaa..." (voix d'anges avec lumière divine venant d'en haut)
Voilà ENFIN une extension remplaçant convenablement les anciens groupes d'onglets disparus avec les persona.
Que son créateur en soit béatifié !
Plus sérieusement enfin une extension qui permet ET de trier les onglets par groupe ET de cacher ceux des autres groupes.
Merci merci merci merci.
Merci.
Merci.

edit:@Coconutsales:
TRY "open manage groups[STG plugin]" by Drive4ik
you can put it on the right.

Great Tab Group!
enjoy manage groups in Popup, Panorama, sidebar.
switch quickly and work well.
careful in many detail

Got so much on the go that I couldn't go back to living without this extension!

Does Simple Tab Groups supports Tab Session Manager? What should I do if FF suddenly crashes? Will Tab Session Manager restore all my tabs AND GROUPS?

Hey, thanks for making this app, it's very useful !

If you have some time, it would be nice if there was a way to move several tabs from one group to another or even merge 2 groups.

Also if you could implement a folder/file system where you can make a group of groups.