This is one of the most needed and useful addons I use. However, there is no data safety when installing a OneTab update!

This is true:
"Data safety
Don't worry about closing OneTab - you will not lose your stored tabs. Even if you close all tabs and exit your web browser and restart your computer, the tabs you have stored in OneTab will not be lost."

However the contents of OneTab totally disappears each time it is updated. Surely there is a way to fix that problem.

All I can suggest is that instead of autoupdates, do manual, and IF there is a OneTab update, then first export the content, so it can be imported after the update. Do not forget to do so if you do not want your content to disappear.