hello nev23, i doubt that this will be possible within firefox as this sounds rather like a task that has to be handled on the level of the OS. there seems to be a payware named IndieVolume that apparently offers this feature for windows xp (i haven't tried it myself, so this isn't really a recommendation)...

I have spent an hour tinkering with IndieVolume and it simply doesn't work. It does nothing at all. It looks like it provides a solution to my problem, but it has no effect at all.

Sorry, I've got it now. The plugin container needs redirecting, not just Firefox. In order to control all audio coming through Firefox, how many different plugin containers will I need to redirect?

if the program doesn't work well with multiple plugin-container.exe instances you could disable them generally as it's described at http://kb.mozillazine.org/Plugin-container_and_out-of-process_plugins

It now looks like a motherboard problem. I have built another computer that is a close copy of the one at the radio station, except for the motherboard. Zara behaves as expected, not changing its audio setup back to default devices when restarted. And on the radio station computer, IndieVolume made the erratic behaviour worse, and required me to uninstall it and also reinstall Zara, after stripping out all Zara registry keys because a simple uninstall-reinstall didn't get me back to the original situation.

So thanks for your help getting me to realise that it wasn't a Firefox problem.