It is not the same thing, but have you looked at
tab groups in Safari 15.4 to see if they might satisfy your need?
In the complete overkill department,
DevonTHINK 3.8.2 can have a
Group of Bookmarks and then create a
Table of Contents of that
Group which would be pretty close to what I
think you are describing, although the default browser would be the version of Apple's webkit that is built into DevonTHINK.