It f*cked up all saved tabs. Second time. I haven't been making backups too often due to info from forums that previous version is free from this problem. The last version came without any wornings and deleted all tabs.
Unfortunatly the method, that was mentioned in commets.
If you use it, everytime make a backup via html doc.
Now I have to go - I need to get through the entire history for at least a week. F************ck!
F*ck the devs.

It was working ok, but latest version erased all my save tabs.
Luckily as explained by a previous comment, restoring the previous version bring the saved tabs back.

Developer answered, and they fixed it. Read the "About" section for an explanation on how to fix it.

Giving 4 stars now ;)

ALL of my tabs was gone.

UPDATE. After upgrade to version 1.27, my tabs are back.

DO NOT UPDATE TO 1.26, I LOST ALL MY TABS! 💣

Downgrading to 1.25 solved the issue
https://addons.mozilla.org/en-US/firefox/addon/onetab/versions/
https://addons.mozilla.org/firefox/downloads/file/767772/onetab-1.25-fx.xpi

Helps reduce CPU + RAM usage on my terrible laptop, incredibly useful, thank you for making it.

Lose all tags with great probability, you don't know when it will happen

Two days ago I would have given this a 5-star rating. However, today ... all my saved tabs are gone! I see it's been two years since this has been updated and, given the number of unanswered complaints, I am guessing this won't be fixed. I am using Ubuntu 18.04 and can't find any help to recover the tabs. Oh well, moving on to another I guess. Too bad. It was great once.

I love it!

STAY AWAY!!! This shit just deleted about 200 tabs for me by moving only 47 tabs to the OneTab window when using the send all tabs from all windows to OneTab feature, closing all tabs one by one and then closing the windows, so restoring the windows with Firefor built in feature only brings back ONE tab per window and not even all windows (3/10). As you can imagine i'm quite upset by this.

I absolutely LOVED this extension! That is until it LOST ALL of my tabs!! 😡 I would literally PAY for syncing capabilities.

It loses your tabs! Beware of this possiblity. It was last updated 2 years ago so may be incompatible with the latest firefox.