From MozillaZine Knowledge Base
The QuickTime Player for Windows makes it possible to view and listen to many different types of media, including audio and video "embedded" in web pages.
Important: QuickTime 7.1.x is currently on the Add-ons Blocklist and does not work in Firefox 3.
If you are installing Quicktime Alternative, make sure that the "Firefox/Mozilla/Netscape/Opera Plugin" is checked when selecting which components to install.
If your version of QuickTime doesn't include an "Update Existing Software" feature or if the new version isn't offered this way, you can install the new QuickTime version "over the top" of your current installation. This will automatically replace the old version with the new one.
You can verify the QuickTime version your Mozilla browser is detecting in about:plugins or, starting in Firefox 3 and SeaMonkey 2, in "Tools -> Add-ons (Add-on Manager) -> Plugins".
QuickTime audio settings
QuickTime browser settings
In Windows XP and below you can configure the MIME types that the QuickTime plugin will handle in QuickTime Preferences. The MIME types selected will be listed under the QuickTime plugin in about:plugins and both downloaded files and embedded content with those MIME types will be handled by the QuickTime plugin. Starting in Windows Vista, QuickTime preferences no longer include configurable MIME settings for the browser plugin; only file extensions associated with the QuickTime application can be changed.  
As an alternative, you can prevent downloaded files from opening in the QuickTime plugin by changing your (Helper) Application settings (see File types and download actions for details). However, the browser plugin will still be used for QuickTime content embedded in web pages based on the MIME types shown in the about:plugins list. In Firefox 3 or SeaMonkey 2 and above, you can also disable the plugin in the Add-on Manager Plugins list if you don't want the QuickTime plugin to handle embedded content.
Configuring QuickTime MIME Types
The following applies to Windows XP and below.
This is where you enable the QuickTime browser plugins for different types of media. (Don't go to the File Types preferences by mistake; those are for Windows file associations.)
Close and reopen the browser to effect the changes.
Suggested MIME Types:
Important: When selecting the media types you want QuickTime to handle under MIME Types, do not select "Flash media", if present, as this will cause a conflict with the Flash plugin. Note that, as of QuickTime 7.3.1, the Flash media handler has been disabled and "Flash media" should no longer be listed. 
Resetting the QuickTime browser plugins
Sometimes the QuickTime plugins are not detected properly, even though you have selected the right media types in the QuickTime browser settings, as described above. You may receive the error, QuickTime Unavailable. The QuickTime Plug-in requires the QuickTime System extension version 5 or later. If you enter about:plugins in the address bar, you may also see plugins listed from different versions of QuickTime  or other inconsistencies. To reset your QuickTime browser plugins: 
Related bug reports