I tried it a month or so back when it was first released. Seemed really basic, which I guess is fine for an initial release.
My issue with it at the time was there wasn’t any way to import any existing comics I already had.
I’ll need to spin it up again and see what has changed since I last looked at it.
There are no extensions that do it exactly like Chrome does with the collapsible groups in the tab bar, Firefox doesn’t support it.
There are a few extension for grouping tabs that other people have mentioned though.