doc: update config option for firefox support

[cch5ng]

resources

• http://caniuse.com/#search=speech%20synthesis
• http://kb.mozillazine.org/About:config

[cch5ng]

even after updating the config, the react templates are not rendering (appears that the feed data has downloaded)

may be related to:

• https://github.com/meteor/meteor/issues/5179
  • however, I'm unclear of a resolution

issue found on *.meteor.com deployed site check if there's any difference on locally hosted application

[cch5ng]

on locally hosted application

• the feeds render visually
• the play/pause buttons appear to have no effect
  • even after the firefox configs have been updated
  • double check whether speech synthesis api syntax is correct for firefox (there might have been a difference in prefix with chrome but it appeared to with in chrome fine without the webkit prefix)