Seems to work and has a little progress page. Opens tabs in the active window, but they are unfocused tabs and do not really disturb browser use while its working. Seems to do a better job not breaking my firefox compared to a previously tried extension.

Simple and works... slowly. Gotta delete history for past hours though. Not an extension we need to use too much anyway.
I don't like how it doesn't have filters (to ignore moz-dev urls and scripts), doesn't group based on domains, and literally opens and closes in the visible tab in active window. At least it would be better if it could be in an inactive window.