You might try going to about:config in the address bar and see if browser.tabs.groups.enabled is set to true. If so, change the value to false. Looks like the default value is true.

That should make the feature go away.

Thank You Mark !

As an ex-fortran programmer(!) I cannot find 'about:config in the address bar '. Really sorry but sometimes the technology beats my ancient brain. Mike

As an ex-fortran programmer(!) I cannot find 'about:config in the address bar '. Really sorry but sometimes the technology beats my ancient brain. Mike

What this means is to type that string, 'about:config', into the address bar and hit enter.

That will open a special screen warning you about editing advanced settings, which after being dismissed, will show a search bar (as opposed to every single setting in Firefox). You can filter the results by setting type (boolean, string, integer), and the search seems like simple string-contains-string (i.e. no reg-ex'ing required).