fx-compat: <split-menu-button>: Use margin instead of absolute height

And add space to the right of the button text. Should look right on Linux now.

Fixes #3041

chrome/content/zotero/elements/splitMenuButton.js

@@ -71,6 +71,9 @@
 			let hiDPISuffix = window.devicePixelRatio > 1 ? '@2x' : '';
 			let frag = document.importNode(
 				MozXULElement.parseXULToFragment(`
+					<vbox>
+						<box anonid="dropmarker-separator"/>
+					</vbox>
 					<hbox align="center" anonid="dropmarker-box">
 						<image src="chrome://zotero/skin/searchbar-dropmarker${hiDPISuffix}.png" width="7" height="4" class="split-menu-button-dropmarker"/>
 					</hbox>

chrome/skin/default/zotero/overlay.css

@@ -721,12 +721,10 @@
 	overflow: hidden;
 }
 
-.split-menu-button [anonid="dropmarker-box"]::before {
-	content: '';
-	display: inline-block;
-	height: 12px;
+.split-menu-button [anonid="dropmarker-separator"] {
+	-moz-box-flex: 1;
 	border-inline-start: 1px solid #aaa;
-	margin-inline-end: 2px;
+	margin: 5px 2px 5px 4px;
 }
 
 .split-menu-button .split-menu-button-dropmarker {