Narrate doesn't list some Windows voices installed from voice packages
Install English (Australia) Windows language pack and reboot. Install Firefox. Install Windows voice package English (Australia) which adds the "Microsoft Catherine" and "Microsoft James" voices (see screenshot) ["Microsoft James" actually has a UK accent not Australian but that's a Windows bug]. Reboot or logout and login again. The Firefox Narrate voice options list only has "Microsoft David Desktop - English (United States)" & "Microsoft Zira Desktop - English (United States)" - "Microsoft Catherine" and "Microsoft James" are not listed.
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I also tried adding the "English (United Kingdom)" voice package and this did actually result in the "Microsoft Hazel Desktop - English (United Kingdom)" voice being added to the Narrate voice list. The other UK voices "George" & "Susan" which appear in the Windows Speech Voices list did not appear in Firefox Narrate however (see screenshots).
This same issue was raised on Reddit (https://www.reddit.com/r/firefox/comments/al36uj/) and as irvinm66 speculates it seems that Firefox may only be recognizing so called "Desktop" type voices. Only the installed Desktop" type voices appear in my Windows registry at "Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech\Voices\Tokens" however all installed voices appear in the registry at "Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Speech_OneCore\Voices\Tokens" so perhaps the wrong registry key is being used.
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A search for HKEY_LOCAL_MACHINE in this file seems to confirm this.