Fix firefox fingerprint protection.

ansible/roles/firefox/tasks/peruser.yaml

@@ -10,12 +10,21 @@
   register: firefox_about_config
 
 - name: Configure Firefox about:config
+  when: item[1].value != None
   lineinfile:
     path: "{{ item[0].path }}"
     regexp: '"{{ item[1].key }}", [^")\n]*\)'
     line: 'user_pref("{{ item[1].key }}", {{ item[1].value | to_json }});'
   loop: "{{ firefox_about_config.files | product(firefox_config | dict2items) | list }}"
 
+- name: Configure Firefox about:config
+  when: item[1].value == None
+  lineinfile:
+    path: "{{ item[0].path }}"
+    regexp: '"{{ item[1].key }}", [^")\n]*\)'
+    state: absent
+  loop: "{{ firefox_about_config.files | product(firefox_config | dict2items) | list }}"
+
 - import_tasks: tasks/peruser_freebsd.yaml
   when: 'os_flavor == "freebsd"'