Hmm, either the files are different -- for example, different compression CODEC -- or perhaps the way they are identified/served is confusing Firefox as to what they actually.

So if you don't mind a little nerdy look under the covers...

You can use the Web Console and Network Monitor in the Developer Tools to gather more information. To open the console, use Command+Alt/Option+K. This should show any error messages or warnings gathered since the tab was opened, but just to be sure, reload the page bypassing the cache (Command+Shift+R). Then try playing the media and see whether any more detailed error information is available there. Also, switch over to the Network mini-tab and look for the media there. You can click the row for the MP3 to see additional information about failure to load, and to check the Response Headers, which typically specify to Firefox what kind of media it is.

Let us know what you see.

Much appreciate your thoughtful, detailed response. Sad to say, it's too far over my tech-head capabilities. In my old age, learning comes very slowly, especially in fields where I'm unfamiliar with the basics.

I will save your directions and if I can get some help from someone hereabouts, maybe take a crack at it.

For the time being, I'm just going to add at the top of the page a "We apologize for technical difficulties: Firefox browsers do not play the music files on this page" (or something like that, with a little humor). All other browsers will play them.

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required)?

Hope this helps: https://www.gatherthepeople.org/styled-7/page-3/

Hmm, for some reason Firefox wants to play the file using a VIDEO tag and that fails. Forcing an audio tag works.

data:text/html,<audio src="https://www.gatherthepeople.org/resources/Media/GTP_02_Ma_Tovu.mp3" controls>

They play fine for me in my regular profile, but not in a new profile. It seems that the requests are the same, and in both cases the server says the content size is 8470593, but the number of bytes delivered in the first response is a bit different. In my regular profile, Firefox requests the rest of the file while in the new profile, it doesn't, it just seems to stop.

I can't find a relevant different in media settings between the two, so maybe it is something related to network settings (or add-ons). ??

I noticed when I right-click a media link and choose Save Link As, Firefox surprises me with a different file extension. For example:

GTP_01_L'shem_Likhud.m4a

That seems like it might be a clue, but I still don't know why Firefox behaves properly in the old profile and not the test profile.