I suggest switching to sidebery. Use the 5.x version from github, but if you do that they have a recipe.

https://github.com/mbnuqw/sidebery/wiki/Firefox-Styles-Snippets-(via-userChrome.css)

IMO the default styles look kinda ugly. I made one that looks very similar to treestyletabs

#root.root {--tabs-activated-shadow: 0px 1px 4px -1px rgba(0,0,0,.5), inset 0 0 0 1px rgba(255,255,255,0.133);}
#root.root {--tabs-activated-bg: #d3ebffff;}
#root.root {--tabs-inner-gap: 6px;}
#root.root {--tabs-indent: 8px;}
#root.root {--tabs-font: .9rem sans-serif;}
#root.root {--tabs-height: 25px;}
#root.root {--popup-bg: rgb(244, 245, 246);}
#root.root {--general-margin: 0px;}
#root.root {--frame-bg: #9a9996ff;}

#root.root {--status-active: #146fd9;}

#root.root {--tabs-border-radius: 4px 0 0 4px;}


/* OLD CSS VARS

*/

.Tab .body{
  border-style: solid;
  border-width: 0px 0px 1px;
  border-color: rgba(154, 153, 150,0.55);
	background-color: #FCFCFC;
}

.Tab[data-discarded="true"] > .body > .fav,
.Tab[data-discarded="true"] > .body > .t-box, 
.Tab[data-discarded="true"] > .body > .ctx {
  opacity: 1.00;
	--frame-fg: red;
}

.TabsPanel .bottom-space {
	height: 0;
}

.TabsPanel .bottom-bar {
	display: none;
}

.Tab .progress-spinner {
	right: 1px;
	bottom: 1px;
}

.Tab[data-loading="true"] .fav-icon {
  mask: radial-gradient(circle at calc(100% - 8px) calc(100% - 8px),rgba(0,0,0,.141),rgba(0,0,0,.141) 8px,#000 8px,#000);
}