Rated 5 out of 5 stars
just updated and read down the reviews (actually the guys' thankfulness) of addon "2 Pane Bookmarks", and found my lovely "tree style tab" also renewed to support ff20. incredible!
(quote "impossible for me to move to a different browser." by stubi.)
Thanks a minion!
(p.s. : if guy like me, like to read down the reviews, one should check a addon called "pageexpand, a new and wonderful addon)