The worst add-on I had on my system for months. Had the same running in Chrome, where it works quite well. On Firefox (72.01, 64 bit) win7 64 pro, it let me lose more than 250 tabs collected over a long time. Hope to retrieve them in my backups, but this add-on should be taken of the list immediately. One star is still one too many.
If the developers want to keep a small fraction of credibility, they should publish a method how to retrieve the lost tabs yet today.

I like it, but there are few issues I encounter:
1. doesn't play well with multi-account containers, so my tabs forget which container they were opened in.
2. Clicking the onetab button is a very bad UX. It should take me to onetab or open a menu rather than converting all my tabs and taking me a while to restore the mess it made (see 1).
3. Right clicking to get through several layers of menus to convert a single tab is a bad UX. Please give the option to right click on the tab itself to convert just that tab.