Used this extension for ~2 years, found it to be essentially OK, not perfect but got the job done through some unintuitive means.

But a couple of times upon closing firefox all of my tab groups were simply emptied. (Might be due to ending firefox with task manager? But mostly didn't notice the issue till a day or so later so can't confirm).

Usually had some pretty recent backups but still very very annoying. Then today the same issue happened, and it turns out inexplicably my last backup was over a month ago and I lost almost a hundred saved tabs.

Cannot recommend this extension in its current state.