This feature was turned on remotely using the study/hotpatch system ([[About Studies]]). Maybe that was disabled, blocked, or just failed on your Firefox. To turn the feature on manually: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. ''More info on about:config:'' [[Configuration Editor for Firefox]]. ''The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.'' (2) In the search box in the page, type or paste '''browser.tabs.groups''' and pause while the list is filtered - Firefox should show several matches (3) If the '''browser.tabs.groups.enabled''' preference has a value of false, double-click it (or click the Toggle button or Reset button at the right end of the row) to change the value to true I don't think you need to change any of the other values (and don't recommend changing things without looking them up).