Oh, I'm yet to hit that problem. My current annoyance is that sometimes it gets confused after Firefox restarts, and I end up with a flat dump of tabs + an unending spam of those tiny warning popups at the bottom of its UI. The few times that happened, I ended up restarting Firefox again to stop the warnings, and then rearranging the flat list of tabs into groups and panels I want them to be in. Fortunately, it's not that big of a chore.