Bug 1267499 - Fix Private Browsing start-page overflows off the window bottom. r=Gijs

MozReview-Commit-ID: F9Do3T8d7R3

--HG--
extra : transplant_source : %BAEaE%D5%A8%C1%86I%7F%B6%3By%C9e%E3%F0d%5C%22

browser/themes/shared/privatebrowsing/aboutPrivateBrowsing.css

@@ -51,12 +51,16 @@ a:visited {
   max-width: 780px;
 }
 
-section.section-main {
+.section-main {
   margin-bottom: 48px;
   -moz-margin-start: var(--icon-margin);
   -moz-padding-start: 24px;
 }
 
+.section-main:last-child {
+  margin-bottom: 0;
+}
+
 p {
   line-height: 1.5em;
 }