fixup! Bug 41916: Letterboxing preferences UI

Bug 42583: Use moz-support-link for the letterboxing preference.

Also update the url.

browser/components/preferences/letterboxing.inc.xhtml

@@ -11,13 +11,11 @@
       data-l10n-id="letterboxing-overview"
       class="tail-with-learn-more"
     ></html:span>
-    <label
+    <html:a
+      is="moz-support-link"
+      tor-manual-page="anti-fingerprinting_letterboxing"
       data-l10n-id="letterboxing-learn-more"
-      class="learnMore text-link"
-      is="text-link"
-      href="about:manual#letterboxing"
-      useoriginprincipal="true"
-    />
+    ></html:a>
   </description>
 </vbox>
 <groupbox
@@ -32,13 +30,11 @@
       data-l10n-id="letterboxing-overview"
       class="tail-with-learn-more"
     ></html:span>
-    <label
+    <html:a
+      is="moz-support-link"
+      tor-manual-page="anti-fingerprinting_letterboxing"
       data-l10n-id="letterboxing-learn-more"
-      class="learnMore text-link"
-      is="text-link"
-      href="about:manual#letterboxing"
-      useoriginprincipal="true"
-    />
+    ></html:a>
   </description>
   <checkbox
     id="letterboxingRememberSize"
@@ -59,13 +55,11 @@
       data-l10n-id="letterboxing-overview"
       class="tail-with-learn-more"
     ></html:span>
-    <label
+    <html:a
+      is="moz-support-link"
+      tor-manual-page="anti-fingerprinting_letterboxing"
       data-l10n-id="letterboxing-learn-more"
-      class="learnMore text-link"
-      is="text-link"
-      href="about:manual#letterboxing"
-      useoriginprincipal="true"
-    />
+    ></html:a>
   </description>
   <description
     id="letterboxingAlignmentDesc"
@@ -110,13 +104,11 @@
       data-l10n-id="letterboxing-overview"
       class="tail-with-learn-more"
     ></html:span>
-    <label
+    <html:a
+      is="moz-support-link"
+      tor-manual-page="anti-fingerprinting_letterboxing"
       data-l10n-id="letterboxing-learn-more"
-      class="learnMore text-link"
-      is="text-link"
-      href="about:manual#letterboxing"
-      useoriginprincipal="true"
-    />
+    ></html:a>
   </description>
   <hbox align="center">
     <label