--- a/browser/themes/linux/syncedtabs/sidebar.css
+++ b/browser/themes/linux/syncedtabs/sidebar.css
@@ -45,16 +45,17 @@ html {
   height: 16px;
 }
 
 .textbox-search-icon {
   background-image: url(moz-icon://stock/gtk-find?size=menu);
   background-repeat: no-repeat;
   width: 16px;
   height: 16px;
+  display: block;
 }
 
 .textbox-search-icon[searchbutton]:not([disabled]) ,
 .textbox-search-clear:not([disabled]) {
   cursor: pointer;
 }
 
 .item.client .item-twisty-container {

--- a/browser/themes/osx/syncedtabs/sidebar.css
+++ b/browser/themes/osx/syncedtabs/sidebar.css
@@ -66,24 +66,19 @@ html {
   border-bottom-left-radius: 2px;
   background-color: #FFF;
   color: #000;
   -moz-user-select: text;
   text-shadow: none;
 }
 
 .search-box.compact > .textbox-input-box > .textbox-search-icons > .textbox-search-clear {
-  width: 11px;
-  height: 11px;
-  background-image: url(chrome://global/skin/icons/searchfield-small-cancel.png);
+  background-image: url(chrome://global/skin/icons/searchfield-cancel.svg);
   background-repeat: no-repeat;
-}
-
-.search-box.compact > .textbox-input-box > .textbox-search-icons > .textbox-search-clear:active:hover {
-  background-position: 11px 0;
+  background-size: 11px 11px;
 }
 
 .search-box.compact > .textbox-input-box > .textbox-search-icons > .textbox-search-icon {
   display: none;
 }
 
 .search-box[focused="true"] {
   -moz-border-top-colors: -moz-mac-focusring -moz-mac-focusring #000000;

--- a/browser/themes/shared/syncedtabs/sidebar.inc.css
+++ b/browser/themes/shared/syncedtabs/sidebar.inc.css
@@ -118,16 +118,21 @@ body {
 
 .textbox-input-box {
   display: flex;
   flex-direction: row;
 }
 
 .tabsFilter {
   flex: 1;
+  /* min-width of anything to override the implicit "-moz-min-content" value.
+     0px is safe as the sidebar itself has a constrained size meaning we will
+     never actually hit this minimum
+  */
+  min-width: 0px;
 }
 
 .sync-state > p {
   padding-inline-end: 10px;
   padding-inline-start: 10px;
   color: #888;
 }