When you select:

Firefox->Preferences (in Mac)
Tools -> Options -> Applications (in Windows)

you'll see a list of helper applications that Firefox uses for opening other content. In that list in the Action column you'll have the ability to select an action for a specific file type. For .mov type files, if your Quicktime Player appears as an option in the list, try selecting "Use Quicktime Player" and then restart your browser.

If it does not appear in the list, try selecting "Always ask", and then when asked later, you should be given an option "Open with ..." and then can navigate to the Quicktime program to select it on your computer.

Once you have used an external program like that, Firefox will remember your selection and it should appear in the Options -> Applications column. Then when you click on a link for a file with the .mov file extension, the file will open in the Quicktime Player rather than trying to use the plug-in.

Some sites look for evidence in your browser of the Quicktime plug-in and won't serve .mov files if the plug-in isn't detected, which is unfortunately bad programming on their part, but you can leave feedback on those sites.

edited for Mac/Windows path differences

Application content only available in Preference window. .mov type not available as a choice. See image below.

Go to the Apple test page and click on one of the .mov files on the right-hand side of the page, and then follow my directions above for selecting a default file handler.

scti: I am not following what you want me to do. I do not see .mov files on the right hand side of page. The page you sent me to has the below image. The downloads are samples. I really am confused .

Thanks for trying to help. It is appreciated.

The 2nd and last items in the list are. mov files. If you click on the download link for the 2nd file (on the Quicktime icon that is entitled download), you should be presented with a prompt that asks which program you wish to use to open the file. Then follow the directions above, and report back on your success or further need for help here.

sctl, on Mac it is "Firefox->Preferences" not "Tools->Options" ;)

sctl: I did what you asked and ended up with (see image below):

I think the problem is that sctl is trying to help me but he isn't using a Mac.

Mea culpa, those files are indeed zipped and not what you need for testing. Try: http://people.sc.fsu.edu/~jburkardt/data/mov/mov.html
This page has some test files you can open as described above.

Ok. Here is the result. Most samples played using Quicktime except for MPEG-4 (see image 1). Plugin Finder found that there were no suitable plugins found. (see image 2).

I think that Firefox can not play MPEG-4s no matter what we do. I will probably have to use Safari to use Apple Movie Trailer page.

Thanks both sctl and James for your help.

JDB38

Sorry for the Mac/Windows confusion. Please report back if you have any further progress.