Upgrading Firefox to version 20

Looks like it was downgraded to 19 by mistake in a recent commit:
https://gerrit.wikimedia.org/r/#/c/55266/6/config/config.yml

Change-Id: I79d8a51f9b3ef4acdf79f3f6af0d9c9a0c560b1c
Merge "Upgrading Firefox to version 20"