You might try going to about:config in the address bar and see if browser.tabs.groups.enabled is set to false. If not, change the value to false.

That should make the feature go away.

Update: corrected my response.

The about:config workaround works for me. I am sure it is a nice feature for some... But for other users like me, and probably older either, it is a true pain in the neck when it groups automatically things, you don't notice it or delete the group by error, and end up searching for the lost tab(s) re-opening it again. Especially boring and site-breaking when it is a secured site with a specific 1-time security (ex, some gov sites), or uses various containers, etc... In short, not friendly when not wanted/controlled (without going under the hood, but thanks to MarkRH). The option to turn on/off should be added in the settings, asap, under General / Tabs.

Thanks, I can't believe this is not in settings. I sometimes rearrange the order of my tabs and it automatically grouped them and can't believe someone thought this was agood idea