Thanks! The extension is Sidebery. This single extension has changed my browser workflow more than the whole Vivaldi browser that I used before Firefox. I have around 350 tabs open, neatly organized into panels, groups and subgroups. Firefox unloads tabs, so it has (almost) no effect on performance. It’s super-customizable, and integrates with Firefox’s Multi-Account Containers. It also takes snapshots of all of your open tabs for easy backup or migration. The workflow is amazing. I would think that I would spend much time organizing the tabs, but it’s actually much faster than managing the tabs the normal way: I can open 20 tabs in a hurry and when I’m done, just close them all or just collapse them. I can’t recommend it more.