Tree Style Tab plugin provides ability to operate tabs as "tree" and tabs opened from the current tab are automatically organized as "children" of the current one. This is a very useful plugin for Firefox users.
But on Firefox version 69, the tabs are displaying on top header bar even after enabling Tree Style Tab plugin in Firefox.
This is because of the changes of Firefox browser.
Mozilla plans to land a change in Firefox 69 that disables the loading of userChrome.css and userContent.css by default to improve performance. The files userChrome.css and userContent.css are used to modify content of webpages or the browser itself using CSS instructions.
This option to do so is not removed but Mozilla plans to make it opt-in instead of opt-out. So we can enable it manually in-order to solve the problem.
Step1: Load about:config in the Firefox address bar.
Step2: Confirm that you will be careful.
Step3: Search for toolkit.legacyUserProfileCustomizations.stylesheets using the search at the top.
Step4: Toggle the preference. True means Firefox supports the CSS files, False that it ignores them.
Step5: Reopen Firefox.
Now the Tree Style Tab plugin will work perfectly and the tabs will not be displayed at top header.