Those voices come from Microsoft. I have yet to see more voices unless you add more language options from Microsoft updates. Most software use the Windows default unless those packages come with additional voices.

I installed other voices from Microsoft yesterday but they did not appear in Firefox. I loaded up firefox today and now the other voices from Microsoft came up for some reason in Firefox. I'm only looking for voices in English and want to know where I can download them from.

Then it could mean that FF doesn't support extra voices at the current time or doesn't recognize the new voices. Did you restart the computer when you added more voices from Windows update? If not then you might have to ask more of Mozilla support to see if there is something that needs to be switch on in FF to recognize more voices in English. But voices usually is set by Windows O/S that I know of. I tried that before so that another voice narrating program would work but it only recognized the voices that were from Windows and that was Zira, David, Ann and when I added more it didn't work. So I was left with Windows default.