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.