How can I prevent tree style tabs from opening tabs under parent tabs?

While Tree Style Tab is meant to work in automatically built trees based on conditions related to how you open tabs, you actually can disable it automatic functionality while keeping manual tab grouping possible by selecting one or more tabs and dragging them over another tab, this can be done. All you have to do is open addons menu with hamburger menu or opening a tab and typing "about:addons" from their looking in Extensions section will Tree Style Tab Addon, select it and select Preferences followed by scrolling to bottom & selecting "Development" section, them find "All Configs" & look for "autoAttach" & "syncParentTabAndOpenerTab" and click checkmark and disable both. Though if you mean how can I stop trees from being collapsed by selecting another tree with atleast one indent or following opening a child tab under a root level tab, Scroll Up to Tree Behavior, and select checkmark, "When A Tab Gets Focus, Expand It's Tree & Collapse Others Automatically" for the former and "When A New Tree Appears, Collapse Others Automatically" for the latter, and select both if both are of trouble to you. Though there is a related "Tree Behavior" that unfolds collapsed trees called "When A Tab Under A Collapsed Tree Gets Focus, Expand It's Tree Automatically" which is useful is you switch through tabs using the browsers default tab bar, or any other means allows you to select tabs inside tree without expanding it.