If you type about:config into the address bar, press Enter, then accept the warning, then search for Media, you should see several prefs called "media.comething.enabled". If you switch these to false does that do what you are wanting?

Not that I can figure out. For example, if I have an embedded link to a .mp3 file (say, for a podcast) using the audio tag, then firefox loads the file in the background. There is nothing in the media.*.enabled that suggests that it would stop that from happening. There is no mp3 item in that list nor is there anything about preload defaults (set to no when it is 'auto' in the html). The other settings for mp3 and flash are set to false in areas I can find.

You can set the media.windows-media-foundation.enabled pref to false on the about:config page to disable the built-in HTML5 media player.

Thank you! Works perfectly.

How do I do this on the mac now? The previous works well for Windows.

Does Firefox play MP3 files in an HTML audio tag on Mac?

A lot of other types can be disabled via one of the media prefs, but if there isn't one for files that still play and disabling media.autoplay.enabled to false also doesn't help then I don't know what else to try.

I thought it did the same as I was seeing in Windows, but it seems to not do it now. The vast majority of the times I run into it I am using Windows.

Thanks for the suggestion. If it comes up, I have a place to start.