--- a/browser/components/customizableui/content/panelUI.inc.xul
+++ b/browser/components/customizableui/content/panelUI.inc.xul
@@ -121,70 +121,75 @@
           <deck id="PanelUI-remotetabs-deck">
             <!-- Sync is ready to Sync and the "tabs" engine is enabled -->
             <vbox id="PanelUI-remotetabs-tabspane">
               <vbox id="PanelUI-remotetabs-tabslist"
                     notabsforclientlabel="&appMenuRemoteTabs.notabs.label;"
                     />
             </vbox>
             <!-- Sync is ready to Sync but the "tabs" engine isn't enabled-->
-            <vbox id="PanelUI-remotetabs-tabsdisabledpane"
-                  class="PanelUI-remotetabs-instruction-box">
-              <hbox pack="center">
-                <image class="fxaSyncIllustration" alt=""/>
-              </hbox>
-              <label class="PanelUI-remotetabs-instruction-label">&appMenuRemoteTabs.tabsnotsyncing.label;</label>
-              <hbox pack="center">
-                <toolbarbutton class="PanelUI-remotetabs-prefs-button"
-                               label="&appMenuRemoteTabs.openprefs.label;"
-                               oncommand="gSyncUI.openSetup();"/>
-              </hbox>
-            </vbox>
+            <hbox id="PanelUI-remotetabs-tabsdisabledpane" pack="center" flex="1">
+              <vbox class="PanelUI-remotetabs-instruction-box">
+                <hbox pack="center">
+                  <image class="fxaSyncIllustration" alt=""/>
+                </hbox>
+                <label class="PanelUI-remotetabs-instruction-label">&appMenuRemoteTabs.tabsnotsyncing.label;</label>
+                <hbox pack="center">
+                  <toolbarbutton class="PanelUI-remotetabs-prefs-button"
+                                 label="&appMenuRemoteTabs.openprefs.label;"
+                                 oncommand="gSyncUI.openSetup();"/>
+                </hbox>
+              </vbox>
+            </hbox>
             <!-- Sync is ready to Sync but we are still fetching the tabs to show -->
             <vbox id="PanelUI-remotetabs-fetching">
               <label>&appMenuRemoteTabs.fetching.label;</label>
             </vbox>
             <!-- Sync has only 1 (ie, this) device connected -->
-            <vbox id="PanelUI-remotetabs-nodevicespane"
-                   class="PanelUI-remotetabs-instruction-box">
-              <hbox pack="center">
-                <image class="fxaSyncIllustration" alt=""/>
-              </hbox>
-              <label class="PanelUI-remotetabs-instruction-label">&appMenuRemoteTabs.noclients.label;</label>
-              <!-- The inner HTML for PanelUI-remotetabs-mobile-promo is built at runtime -->
-              <label id="PanelUI-remotetabs-mobile-promo"/>
-            </vbox>
+            <hbox id="PanelUI-remotetabs-nodevicespane" pack="center" flex="1">
+              <vbox class="PanelUI-remotetabs-instruction-box">
+                <hbox pack="center">
+                  <image class="fxaSyncIllustration" alt=""/>
+                </hbox>
+                <label class="PanelUI-remotetabs-instruction-label">&appMenuRemoteTabs.noclients.label;</label>
+                <!-- The inner HTML for PanelUI-remotetabs-mobile-promo is built at runtime -->
+                <label id="PanelUI-remotetabs-mobile-promo"/>
+              </vbox>
+            </hbox>
           </deck>
         </vbox>
-        <!-- When Sync is not configured -->
-        <vbox id="PanelUI-remotetabs-setupsync"
-              flex="1"
-              align="center"
-              class="PanelUI-remotetabs-instruction-box"
-              observes="sync-setup-state">
-          <image class="fxaSyncIllustration" alt=""/>
-          <label class="PanelUI-remotetabs-instruction-label">&appMenuRemoteTabs.notsignedin.label;</label>
-          <toolbarbutton class="PanelUI-remotetabs-prefs-button"
-                         label="&appMenuRemoteTabs.signin.label;"
-                         oncommand="gSyncUI.openSetup();"/>
-        </vbox>
-        <!-- When Sync needs re-authentication. This uses the exact same messaging
-             as "Sync is not configured" but remains a separate box so we get
-             the goodness of observing broadcasters to manage the hidden states -->
-        <vbox id="PanelUI-remotetabs-reauthsync"
-              flex="1"
-              align="center"
-              class="PanelUI-remotetabs-instruction-box"
-              observes="sync-reauth-state">
-          <image class="fxaSyncIllustration" alt=""/>
-          <label class="PanelUI-remotetabs-instruction-label">&appMenuRemoteTabs.notsignedin.label;</label>
-          <toolbarbutton class="PanelUI-remotetabs-prefs-button"
-                         label="&appMenuRemoteTabs.signin.label;"
-                         oncommand="gSyncUI.openSetup();"/>
-        </vbox>
+        <!-- a box to ensure contained boxes are centered horizonally -->
+        <hbox pack="center" flex="1">
+          <!-- When Sync is not configured -->
+          <vbox id="PanelUI-remotetabs-setupsync"
+                flex="1"
+                align="center"
+                class="PanelUI-remotetabs-instruction-box"
+                observes="sync-setup-state">
+            <image class="fxaSyncIllustration" alt=""/>
+            <label class="PanelUI-remotetabs-instruction-label">&appMenuRemoteTabs.notsignedin.label;</label>
+            <toolbarbutton class="PanelUI-remotetabs-prefs-button"
+                           label="&appMenuRemoteTabs.signin.label;"
+                           oncommand="gSyncUI.openSetup();"/>
+          </vbox>
+          <!-- When Sync needs re-authentication. This uses the exact same messaging
+               as "Sync is not configured" but remains a separate box so we get
+               the goodness of observing broadcasters to manage the hidden states -->
+          <vbox id="PanelUI-remotetabs-reauthsync"
+                flex="1"
+                align="center"
+                class="PanelUI-remotetabs-instruction-box"
+                observes="sync-reauth-state">
+            <image class="fxaSyncIllustration" alt=""/>
+            <label class="PanelUI-remotetabs-instruction-label">&appMenuRemoteTabs.notsignedin.label;</label>
+            <toolbarbutton class="PanelUI-remotetabs-prefs-button"
+                           label="&appMenuRemoteTabs.signin.label;"
+                           oncommand="gSyncUI.openSetup();"/>
+          </vbox>
+        </hbox>
       </vbox>
     </panelview>
 
     <panelview id="PanelUI-bookmarks" flex="1" class="PanelUI-subView">
       <label value="&bookmarksMenu.label;" class="panel-subview-header"/>
       <vbox class="panel-subview-body">
         <toolbarbutton id="panelMenuBookmarkThisPage"
                        class="subviewbutton"

--- a/browser/themes/shared/customizableui/panelUIOverlay.inc.css
+++ b/browser/themes/shared/customizableui/panelUIOverlay.inc.css
@@ -671,20 +671,21 @@ toolbarpaletteitem[place="palette"] > to
 #PanelUI-remotetabs-mobile-promo {
   margin: 15px;
   text-align: center;
   text-shadow: none;
   max-width: 15em;
   color: GrayText;
 }
 
-/* The boxes with "instructions" get extra padding for space around the
-   illustration and buttons */
+/* The boxes with "instructions" get extra top and bottom padding for space
+   around the illustration and buttons */
 .PanelUI-remotetabs-instruction-box {
-  padding: 30px 15px 15px 15px;
+  padding-bottom: 30px;
+  padding-top: 15px;
 }
 
 .PanelUI-remotetabs-prefs-button {
   -moz-appearance: none;
   background-color: #0096dd;
   color: white;
   border-radius: 2px;
   margin-top: 10px;