Add Wall of Browser Bugs entry for #15990

https://bugzilla.mozilla.org/show_bug.cgi?id=1139853
[skip sauce]

docs/_data/browser-bugs.yml

@@ -78,6 +78,16 @@
   origin: >
     Bootstrap#18365
 
+-
+  browser: >
+    Firefox (Windows)
+  summary: >
+    Right border of `<select>` menu is sometimes missing when screen is set to uncommon resolution
+  upstream_bug: >
+    Mozilla#1139853
+  origin: >
+    Bootstrap#15990
+
 -
   browser: >
     Chrome (OS X)