It was a bit of both as I see it. Tab-groups developer quit soon after rewriting the add-on to fit the multi-processing update (which was announced several year

It was a bit of both as I see it. Tab-groups developer quit soon after rewriting the add-on to fit the multi-processing update (which was announced several years in advance I believe, and was anticipated nonetheless), and then right after Mozilla announced the depreciation of XUL in favor of WebExtensions in a year's time, forcing another rewrite and wouldn't be fully functional until the replacement API's are developed. Besides, that replacement API's haven't even now been fully developed points all the same to Mozilla forcing WebExtensions much too quick, as waiting more would have fixed this problem from occurring.