Disable elements of the firefox 57 UI for TreeStyleTab

mannigfaltig said:

I've added a modification that removes any margin at the top:

#TabsToolbar, #sidebar-header {
  visibility: collapse !important;
}

#TabsToolbar {
  margin-bottom: -21px !important;
}

Then go to the hamburger menu > Customization and check "Title Bar" at the bottom left. Then it pretty much looks like FF 56.

I've also changed the style of TSTs in the add-on settings (all the way at the bottom; not in userChrome.css) such that the tabs don't take up as much vertical space and to hide the close box button (and only show it on the mouse-over event):

.closebox {
  display: none
}

.tab:hover .closebox {
  display: block;
}

.tab {
  padding-top: 0px;
  padding-bottom: 0px;
}

You can make .tab even tighter:

.tab .icon {
  padding-top: 2px; /* reducing it by 2px */
  padding-bottom: 2px;
}

:root {
  --tab-height: 22px; /* reducing it by 4px */
}

https://news.ycombinator.com/item?id=15342379

It might not be the direct answer but a suggestion. You don't have to fully disable this section while not all of the sub-sections have shortcuts to bring them to the foreground. Instead you can use this css in ~/.mozilla/firefox/???/chrome/userChrome.css to shrink it.

.sidebar-header,
#sidebar-header {
  font-size: 1em !important;
  font-weight: normal !important;
  padding: 0px !important;
}