Add Wall of Browser Bugs entry for #19390

https://bugzilla.mozilla.org/show_bug.cgi?id=1253468
Closes #19390
[skip sauce]

docs/_data/browser-bugs.yml

@@ -98,6 +98,16 @@
   origin: >
     Bootstrap#15990
 
+-
+  browser: >
+    Firefox (Windows)
+  summary: >
+    Buttons in footer of very long Bootstrap modal not clickable in Firefox 33+
+  upstream_bug: >
+    Mozilla#1253468
+  origin: >
+    Bootstrap#19390
+
 -
   browser: >
     Chrome (OS X)