Add http://bugzil.la/1264125 to Wall of Browser Bugs

Due to the lack of this feature in browsers,
we currently have to implement an annoying setTimeout-based workaround,
which adds an extra layer of complication and probably slightly impacts performance:
e391fcb953/js/src/util.js (L63-L77)

[ci skip]

docs/_data/browser-bugs.yml

@@ -98,6 +98,16 @@
   origin: >
     Bootstrap#18365
 
+-
+  browser: >
+    Firefox
+  summary: >
+    Fire `transitioncancel` event when a transition is canceled
+  upstream_bug: >
+    Mozilla#1264125
+  origin: >
+    Mozilla#1182856
+
 -
   browser: >
     Firefox (Windows)