MB 97: Remove UI cues to install new extensions.

toolkit/mozapps/extensions/content/aboutaddons.css

@@ -766,3 +766,13 @@ section:not(:empty) ~ #empty-addons-message {
 .permission-header > .addon-sitepermissions-host {
   font-weight: bolder;
 }
+
+/* Mullvad Browser discourages users from installing extensions */
+addon-page-header .main-search {
+  /* preserving the vertical space in order to keep the title aligned */
+  visibility: hidden;
+}
+
+#help-button, #empty-addons-message {
+  display: none;
+}