fixup! Bug 32308: Use direct browser sizing for letterboxing.

Bug 42405: Fix betterboxing + findbar horizontal bounce if the scrollbar is not an overlay.

browser/base/content/browser.css

@@ -15,8 +15,9 @@
   From Firefox 115 on, .browserContainer layout is flex / column,
   and without this trick the .browserStack's resize observer
   doesn't get notified on horizontal shrinking.
+  See also tor-browser#42405.
   */
-  overflow-x: hidden;
+  overflow: hidden;
   background: var(--letterboxing-bgcolor);
 }