autohide bm and main toolbars: Fix margins in fullscreen mode

Previously if Firefox was in fullscreen mode and toolbars would autohide
(per Firefox normal setting) then this style would cause content area to
be pushed way too far up because of computed margin-top on
navigator-toolbox. This patch makes navigator toolbox use margin
computed from tabs toolbar height.

chrome/autohide_bookmarks_and_main_toolbars.css

@@ -95,7 +95,11 @@ See the above repository for updates as well as full license text. */
   margin-bottom: calc(-1px - var(--uc-bm-height) + var(--uc-navbar-height)) !important;
   z-index: auto !important;
 }
-
+@media (-moz-bool-pref: "browser.fullscreen.autohide"){
+  :root[sizemode="fullscreen"] #navigator-toolbox[style*="margin-top"]{
+    margin-top: calc(1px - var(--tab-min-height) - 2 * var(--tab-block-margin)) !important;
+  }
+}
 /* Uncomment the next part to enable compatibility for multi-row_bookmarks.css
  * This would break buttons placed in the toolbar,
  * but that is likely not happening if you are using multi-row setup