Fairly easy to add urls to a group, the settings popup disappears if you click outside of it making it tedious to copy/paste urls though. Ability to add multiple groups of urls is nice. One feature I'd like to see is keyboard shortcuts to open groups.
One of the addons I used to open multiple urls was not compatible after the new Firefox update (Firefox Quantum). Tried this one out and it's actually 100x better! No issues unlike the other reviewer having the sets disappear.
I have to check 10 websites daily to keep on top of things, so this makes my life so much easier. The other addon I tried before this one did not save the urls so I had to paste them in every day - tedious!