It worked great until I collected about 25 000 tabs in total.
Now I can't properly open OneTab page with saved tabs because it takes too much time to load.

Better approach - just bookmark all tabs to some folder and close all of them (FF has bult-in functions for both tasks),
It works fast and you can sync your saved tabs using any bookmark sync addon (there are many of them).

This add-on will be much better if:
1) It doesn't open one page with ALL saved tabs (too slow!)
2) It [optionally] uses bookmarks for storage (free sync and other stuff)