Sorry that you lost your tabs. This extension uses `localStorage` (https://developer.mozilla.org/en-US/docs/Web/API/Web_Storage_API) to save URLs for each tab. It is possible that this got cleared somehow. According to the linked docs, `localStorage`: "Stores data with no expiration date, and gets cleared only through JavaScript, or clearing the Browser cache / Locally Stored Data"

If you submit an issue (https://github.com/avbhatt/tabs/issues) with some more information about your Firefox configuration, especially any other extensions that you have installed that interact with `localStorage` I can try and replicate this problem on my end.

Another thing you can look for is the "Saved!" flash after clicking the "Save" button. This means that each tab has been successfully saved. For additional confirmation that tabs have been saved, try clicking "Restore" right after saving. This will open a window with a list of your tabs.