PRO--
Vertical tabs provided by Tree Style Tabs is a wonderful idea. Despite Mozilla's rush to Quantum, at least Piror has provided a version that works (after a fashion) with FF Quantum

CONS--
1. Many of us who use Tree Style Tabs regularly have a great need for its companion extension, "Open Link in New (Tree Style) Tab". OLNT is available for pre-Quantum FF, and works fine, but is not available (yet?) for Quantum.

2. Under pre-Quantum FF, we could create a bookmark of all open TST tabs. Under Quantum, however, we are told Quantum has a bug which prevents our creating a bookmark without prior FF permissions, and are told to use the TST toolbar button to grant ourselves permission to create bookmarks. However, the toolbar button has no explicit control to grant permission, so we still cannot create a FF bookmark of TST vertical tabs.

3. Many of us expected that when we installed the Quantum version of TST, the top-most, horizontal tab bar would disappear, as it always did in pre-Quantum FF. But no-- the horizontal tab bar remains, for some inexplicable reason.