I just don't use tab grouping (yet) and I don't want this particular auto-group feature which I keep triggering by accident
By "auto-grouping" you mean that it groups when you drag and drop, but you still want it available in the context menu? You could set browser.tabs.dragDrop.createGroup.delayMS in about:config to a very high number - then you'll have to hold one tab over the other for a long time before it actually offers to group them.