In version 49, Firefox added a text-to-speech function to its reader mode. On Windows it works well with Microsoft-provided voices (namely Microsoft Zira Desktop or Microsoft David Desktop), but other voices cannot be used as they do not show in the list.
Those voices I would like to use are provided by third parties and are SAPI5 compatible. I tried to fiddle with Firefox settings in 'about:config' and found nothing interesting except for narrate.filter-voices, which allows to show all voices, irrespective of the language.
Is there a way to make SAPI5 voices show up in Firefox ?