I finally solved a couple of problems that were bugging me
with recent Firefox. The solutions may be useful to some.
Two thing: folder icons in the bookmarks menu and the
default scrollbar that's hard to ssee. The "thumb" actually
blends in perfectly with the background of slashdot.org. It
seems to be another one of those designs developed by people
who only use cellphones and not computers.

Icons:
Make a 16x16 PNG, name it fol.png and put it in the chrome folder.
Add this to userChrome.css:

#PlacesToolbarItems toolbarbutton[container="true"] > .toolbarbutton-icon,
#PlacesToolbarItems menu[container="true"] .menu-iconic-left,
:-moz-any(
#PlacesToolbarItems,
#PlacesChevronPopup,
#BMB_bookmarksPopup,
#bookmarksMenu) menu[container="true"]:not([tagContainer="true"]) >
..menu-iconic-left,
/* Standard folder -- in Sidebar, Library, Add/Edit Bookmark dialog */
:-moz-any(
#bookmarks-view,
#editBMPanel_folderTree,
#placesList) treechildren::-moz-tree-image(container),
#editBMPanel_folderMenuList > .menulist-label-box > .menulist-icon,
#editBMPanel_folderMenuList menupopup menuitem {
list-style-image: url(fol.png) !important;
}

For the scrollbar colors, go to about:config -

ui.themedScrollbar
ui.themedScrollbarInactive
ui.themedScrollbarThumb
ui.themedScrollbarThumbHover
ui.themedScrollbarThumbActive
ui.themedScrollbarThumbInactive

Those are all string values that must be added to about:config.
Since the new scrollbar design changes when used, there are
settings for both. If you don't want the scrollbar to hide normally
then the first two items can be set the same to spec the background
color, then set the next 4 the same color to spec the scroller.

I also set the thickness for scrollbars, but now I can't
find it on Win10. That may have only been in Linux, where the
scrollbar was very thin.