Why did all my tabs disappear

Probably because they got put into a Tab Group, a new feature that's driving me crazy. Look for a small tab with a solid color, right click, look for something like "ungroup tabs" Note to Firefox: this is not an intuitive thing, not a good upgrade, you need to make it opt-in, or easier to opt-out. Advanced users: about:config browser.tabs.groups.enabled change value to false