For those who lost their data:
Just close and restart the addon.
(You don't even need to close Firefox.)

I've been using One Tab for a while, and I love it!! After the update on 16 Sept however, I opened my computer to find all the sites listed on my One Tab had disappeared, and the icon wasn't showing properly on the toolbar.
GOOD NEWS!! After I read some reviews, saying they disabled the add-on, restarted, then enabled it again, everything was restored!! Yayyyyyy!!
5 Stars!! ⭐⭐⭐⭐⭐
To all those users who had the same problem and bagged it out in a review, I suggest you try the above fix, and then change your review!! 😊

I lose everything in a moment. Installed on 3 of my 6 profiles, after restart the onetab tab show nothing and the icon button doesn't respond. I recover 2 of 3 profiles tabs with a lot of work.

Superb addon. But after update it stopped working. I disabled it, restarted Firefox, then enabled it. So everything fine again!

lost,stop working,great.

well, this add-on was great... until it just stopped working and lost all my tabs. useless piece of garbage. I would have saved those tabs elsewhere if I had known this add-in was trash 🙃

Con la última actualizacion de septiembre, se atenuó el icono y no funciona. La solucion es desactivar la extensión, cerrar el navegador y al volverlo abrir aparece todo de nuevo. Años usandola y es perfecta esta extensión.

Unfortunately became unusable due to the last update. Workarounds didn't work reliably for me and shouldn't be necessary in the first place.

Releasing an update in this state is a massive oversight.

This is a great tool. With the updates mid Sept. 2020 its toolbar icon grayed out and the saved tabs was blank. Some people uninstalled and reinstalled and said they lost the links. Instead of that I disabled it, restarted Firefox, and re-enabled, and restarted Firefox - the icon and links were all back.

Awesome, I love this addon 😍

As far as the latest update, it basically removed the add-on from Firefox. I followed what Justin said from another review, but for Mac OS : /Library/Application Support/Firefox/Profiles/08nnu9pm.default/extensions > open "Extension@one-tab.com.xpi" with the Firefox application, it'll ask you to reinstall. Now the add-on will be back, and running normal.

Lost my data due to reinstalling of the extension. Other features work well, but the main purpose is to save the data, not to lost it.

Overall it's a good extension but I've had trouble getting my tabs back twice now. This ext would get 5 stars if there was the option to export automatically. As a developer, I don't see why this would be a difficult feature to add. After the first time, I started exporting my tabs periodically so if I did lose any then I probably could remember the most important links from recent memory. This time I lost all my tabs and the icon disappeared. From reading the reviews, I'm far from alone.

To restore my tabs I found a review with instructions. Go to ~/.mozilla/firefox/2tz4pwhp.default/extensions or a similar path in the Files GUI then double-click on a file called extension@one-tab.com.xpi . This opened up onetab, restored my lost tabs, and fixed the missing icon in my browser after giving permissions in the oneTab pop up in my browser and refreshing my OneTab open tab. I am using Ubuntu 18.04 LTS and Firefox 80.0.1 (64 bit). Hope this helps.

Update: This only fixed the problem temporarily. I had to follow the steps again after restarting my computer. An easier step, as was already mentioned in some reviews, is to simply go to Add-Ons in Open Menu (top right corner) and then disable OneTab by clicking the white circle in the blue oval. Click it again to enable OneTab and it should be working again.

發生什麼災難了？在重開 fx 後，記錄下來的網址全沒了……天啊啊啊啊
--- after bugfix
資料回來了！辛苦了！改回五顆星！

Firefox crash one time and this extension stop working too. All of my tabs lost and there's no way to recover them. Is automatic backup really that hard?

All my saved tabs are gone and the disable/enable isn't working for me. OneTab just isn't reliable unfortunately, andnow I've lost months of research and work.

UPDATE:
I am updating again to 5 stars in good faith that the developer has fixed the issue. This really is a great add-on.

Original Review:
Loved this add-on until yesterday. Everything disappeared (page with my links, icon). From reading other reviews it appears that it will work again if I uninstall/re-install add-on, but big bummer to lose my tabs. I got to rely on it a little too much. Now I will know that it is much more of a temporary holding pen.

Giving this a 3 star because its a great product, however their latest update really botched everything. For people looking for a permanent solution unlike the disabling it and re-enabling it (only works until you restart Firefox), you can first back up your tabs using the methods many other reviews have outlined, then scroll down the OneTab page to the More Information section. Here you can click on the link to previous releases, uninstall OneTab and then install version 1.29 and restart Firefox. You may also want to disable automatic updates in the extension settings so it doesn't update again, and this WILL clear any tabs you have loaded in it, so remember to back up the folder before restoring it afterwards. Hope this helps and doesn't get buried instantly.

Hi, as other users also reported, tabs winddow and icon are gone and only reappaer after deactivation followed by reactivation of onetab. Furthermore, I suffer from the missing dark mode. Onetab is the only light window in my firefox which dazzles me. To create a dark skin would be as simple as adding an appropriate css file. On github e.g. an OneTab-Night-Mode is already available but is is not possible to modify an extension due to certificate restrictions by Mozilla. I am afraid I have to abandon the onetab extension for this reason. Nevertheless, the ideas behind this extension is still very good. Regards

After reboot Firefox, button OneTab becomes inactive. https://i.imgur.com/Bp7uMtM.jpg Disabling-enabling helps, but before rebooting.
P.S. Thanks for the correction!

all tabs gone, disenabling and then enabling the addon again resolved the problem