After switching to and from another group for the first time, it crashed while trying to load every single one of the many tabs I had open, despite enabling fast group switching. After shutting down and reloading Firefox, it had somehow managed to reverse the order of all my tabs, and added a long url to each of them that prevented them from loading. In order to uninstall it, I needed to disable fast group switching, which caused another crash. After reloading again, it had somehow turned all of my tabs into a copy of one tab, while simultaneously trying to load every single one of them at once. Eventually I was able to disable this addon, but not before it had destroyed any trace of my tabs.

So the Fast-Switching option uses a poorly-rigged workaround that alters the urls of your tabs, making them unable to load without this addon. And it very clearly DOES. NOT. WORK. Firefox itself starts fine with that many tabs, without trying to load every single one of them.

This addon left me worse than before I tried it. I would have found a way to back up my session, if I had anticipated that an addon could be this poorly made.