Firefox determines the content type from the header sent by the server, not from the .mpg file extension.

In Options > Applications, search for mpeg to pull up the relevant settings..

If you change those, does it work the way you want?

For mpeg I have it set to use VLC which does work. But I am referring to mpg files and not mpeg files. If I uninstall quicktime then mpg does open with VLC, but upon re-installing quicktime it over-rides FF settings somehow. So how do I specify what app I want FF to open mpg files with? Why is there no application setting for mpg files?

Firefox chooses the player based on the content-type header sent by the server, so that's how the Applications tab works, too. Those are the notations in parentheses, such as (audio/mpeg). Normally you would expect a .mpg file to use one of those content types.

Is there one particular site that doesn't behave correctly? Someone could take a look and see what content-type they are sending. (One way you could do this is to install the Firebug extension, but it's overkill for most people.)

QuickTime can be very persistent in wanting to handle (take over) MIME types.

Did you check the file type and MIME setting in the QuickTime Preferences?

• http://kb.mozillazine.org/QuickTime

I should mention I've been a IT tech for 15 years, so I've used firebug before and I will check it out. Cor-el, in the Quicktime preferences for file types and mime types mpg is showing to use VLC and is the box is not checked. Yet quicktime in FF still wants to play mpg files. And when I load a mpg file from IE it plays in VLC, it also plays in VLC when I open a mpg from my desktop.

I tried firebug and it told me nothing but the obvious. Its a mpg file and listed as a audio/mpg content type. I did get things working how I want them (opening mpg in FF with VLC) by going through numerous steps. Uninstall quicktime, install quicktime, let quicktime take over mpg associations, then go to FF options, Application tab and tell mpeg to open with VLC. None of this makes sense as mpg are not mpeg. What is interesting is that out of the blue, when nothing has changed, quicktime will hi-jack the mpg association again (i've had it happen twice now). I didn't open quicktime, I didn't change anything, it just decided to take things over when it wants to. I've spent days on this now, its so frustrating and I hate quicktime with a passion now.

It's possible that in the absence of having its own stored setting for audio/mpg Firefox is checking the Windows registry, and that is why changing the settings in the player applications resolved the issue. For example, I have this key:

[HKEY_CLASSES_ROOT\MIME\Database\Content Type\audio/mpg]
"Extension"=".mp3"
"CLSID"="{cd3afa76-b84f-48f0-9393-7edc34128127}"

For me, [HKEY_CLASSES_ROOT\CLSID\{CD3AFA76-B84F-48F0-9393-7EDC34128127}] is associated with Windows Media Player.

(This all could be more transparent and/or better documented. There is a technical description, but it isn't useful for someone at my level of knowledge: Document Loading - From Load Start to Finding a Handler - MDN.)