I have opted out of the studies then restarted my computer but still have that annoying other bookmarks folder on my bookmarks toolbar. Why won't this annoying uncalled for intrusion go away?
You can set browser.toolbars.bookmarks.2h2020 = false on the about:config page.
You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".
As a footnote, Firefox 85 will have a right-click option to remove Other Bookmarks from the toolbar so you won't have to sacrifice the other features associated with the 2h2020 refresh.
When might that be available?