> Part of the problem is that it's tricky for people to innovate on bookmarks in an accessible way
Actually, I think it should be very simple. We already have Tree Style Tabs and it has Group-Tabs. Just mirror the Tab-Tree with a Bookmark-Tree, where Group-Tabs are folders, done. There is already a tab-tree-addon doing something similar (was it sideberry?), but it's still a bit fizzy and unstable and needs more polishing. I guess firefox itself needs more optimization in that area.
Similar could be done with Tab-Groups, when done good. There used to be a XUL-addon which added a tabbar for groups. Combined with tree style tabs on the side, this made a really awesome interface. I'm still sad that addons can't do this today anymore.