It was a pain for me to piece it together from bits and pieces from the Firefox forums, I’m glad to share and help others avoid the same pain.

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I do that, here’s my userchrome:

:root{ --uc-toolbar-height: 32px; }

:root:not([uidensity="compact"]){--uc-toolbar-height: 38px}

#TabsToolbar{ visibility: collapse !important }

:root:not([inFullscreen]) #nav-bar{
  margin-top: calc(0px - var(--uc-toolbar-height));
}

#toolbar-menubar{
  min-height:unset !important;
  height:var(--uc-toolbar-height) !important;
  position: relative;
}

#main-menubar{
  -moz-box-flex: 1;
  background-color: var(--toolbar-bgcolor,--toolbar-non-lwt-bgcolor);
  background-clip: padding-box;
  border-right: 30px solid transparent;
  border-image: linear-gradient(to left, transparent, var(--toolbar-bgcolor,--toolbar-non-lwt-bgcolor) 30px) 20 / 30px
}

#toolbar-menubar:not([inactive]){ z-index: 2 }
#toolbar-menubar[inactive] > #menubar-items {
	opacity: 0;
	pointer-events: none;
	margin-left: var(--uc-window-drag-space-width,0px)
}
#nav-bar {
  margin-right: 90px;
}

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Raises rent prices, overall cost of living and takes apartments from long term rental market.