Add Wall of Browser Bugs entry for #19626

Refs https://bugzilla.mozilla.org/show_bug.cgi?id=1259972
Closes #19626
[ci skip]

docs/_data/browser-bugs.yml

@@ -108,6 +108,16 @@
   origin: >
     Bootstrap#15990
 
+-
+  browser: >
+    Firefox (OS X & Linux)
+  summary: >
+    Badge widget causes bottom border of Tabs widget to unexpectedly not overlap
+  upstream_bug: >
+    Mozilla#1259972
+  origin: >
+    Bootstrap#19626
+
 -
   browser: >
     Chrome (Android)