Notice for one tab users: Extensions works as intended on firefox quantum, any problem detected on my daily usage. BUT, because Mozilla switched to web extension, all tabs data is stored under AppData/Roaming/Profile folder and inside of \browser-extension-data\extension@one-tab.com folder, as a file named STORAGE.JS.
**If you are using SSD, or using this extension heavily on your pc, I ADVISE YOU TO BACKUP STORAGE FILE DAILY. Because of an unexpected power loss on my computer, I lost my all saved tabs data, and the worse thing is: Because of trim nature on SSD's, there is no way to recover the file itself.

As a future request, the developer should find a more reliable way to store tabs data, more than dumping all tab information into only 1 file, which is a great risk, especially for SSD users. Maybe, some regular backups or twin file system can be implemented on the extension (Take these as example, there must be more reliable ways).

Anyway, I wrote this review with the hurt of my lost data and FOR TL, DR's: I advise you to back up this extension data regularly with external tools.