In Safari the button contains bits of the "delete" text, which does not appear in Mozillla. See the screenshot.
Code changed in jenkins
User: Manuel Recena
JENKINS-35178 Fix close button in Safari (#2378)
(cherry picked from commit d35c7de73b1b78e1aa47470bdff36b0f60884c8d)
Nominated as an LTS candidate, because it impacts UI.
The fix will have 2 weeks of soaking by the RC cut-off