Opening files using plugins
From MozillaZine Knowledge Base
The default action for file types for which a plugin is installed is to open the directly-visited file with the plugin. In Firefox and SeaMonkey 2, you can change the specified download action for files normally handled by plugins, so that the file is opened with a selected external application or saved to disk, instead of automatically opening using the plugin.
If files of a certain type are not being opened automatically with the plugin for that MIME type, as shown in about:plugins, view the list of actions for file content types (MIME types) in the (Helper) Applications or Download Actions listing for your Mozilla application. You can find the file type listings under "Tools -> Options -> Applications" in Firefox and under "Edit -> Preferences -> Browser -> Helper Applications" in SeaMonkey 2. Here, you can change the action to open in Firefox or SeaMonkey using the plugin.
Note that certain file extensions may include multiple entries, one for each MIME type associated with an installed plugin or specified download action. You can see the list of installed browser plugins and related MIME types by entering about:plugins in the address bar. Occasionally, entries may disappear from the list after changing the download action. If closing and reopening the Download Actions or (Helper) Applications window or restarting the browser doesn't restore the missing entry, try resetting the plugin-related browser preferences as described below.
You can set the action for a specific content type (MIME type) to "Always ask" in your options/preferences for (Helper) Applications, as shown here in SeaMonkey 2 and here in Firefox 3. When "Always ask" is selected for files normally handled by plugins like Flash (for SWF files) or Adobe Acrobat Reader (for PDF files), the Opening dialog box will appear when that file type is downloaded and you will be asked to to select an external "helper" application or save the file.
Important: When you choose "Always ask" in your options/preferences for (Helper) Applications for a content type that a plugin can handle, the "Opening" dialog that displays when you download that type of file does not provide a way to open the file in your browser using the plugin. In the case of Flash (.SWF) files (content type application/x-shockwave-flash) , a suitable external application is usually not available. You will need to change the action in your browser's options/preferences for (Helper) Applications, so that the file can be opened in Firefox or in SeaMonkey 2 using the plugin. See File types and download actions for more information.
Problematic browser preferences
Instead of opening a file via the associated plugin, the Firefox "Opening" dialog box may appear, asking you to choose between saving the file or opening it in an external application, even though the file is being sent with the appropriate MIME type and you have a plugin enabled for that type, as shown in about:plugins. If attempting to change the action is unsuccessful, this problem can be fixed by resetting certain plugin-related preferences, as follows :
If you do the above and the problem persists, or if you want to remove all previously-set Download Actions and restore the default actions, close the browser, open the profile folder and delete (or rename) the file mimeTypes.rdf.
Misconfigured web server
Sometimes a misconfigured web server will send a file with an inappropriate MIME type and, instead of the file automatically opening with the browser plugin, an "Opening" dialog box will appear, asking you to either save the file or open it with a chosen external application. This may happen with .wmv (Windows Media Video) files sent as "text/plain" instead of the correct "video/x-ms-wmv" MIME type (see bug 250914, probably related to the fix for bug 220807, outlined here). A Google Groups search shows that this can also occur when downloading .swf (Shockwave Flash) flies and, most likely, other types of files for which a plugin is installed, when the inappropriate "text/plain" MIME type is sent. A similar issue can occur with the "application/octet-stream" MIME type . (Note that the MIME type of the file being downloaded will appear in the Mozilla Suite / SeaMonkey 1.x "Opening" dialog box, but not in Firefox or SeaMonkey 2.) If the problem occurs with some downloads of a particular type file but not others, it is likely that the web server is misconfigured; unfortunately there is nothing you can do in such cases except to contact the website about the problem.
Advanced users can use the Live HTTP Headers to detect the MIME type being sent.
Windows Media Player plugin
The standard Windows Media Player plugin (file name "npdsplay.dll"), when loaded from a direct link to a "Windows Media" file format (.asf, .asx, .wm, .wma, .wax, .wmv, and .wvx), will not appear in the page and display the file like other plugins. It will just appear as a blank page, download the whole file, and then hand it off automatically to the default media player for that file type. In other words, if another player (such as WinAmp or Real Player) is the default player for .wma (Windows Media Audio) , .wmv (Windows Media Video) or other "Windows Media" formats, then direct links to those files will result in your default player opening, even if the Windows Media Player plugin is installed and listed as handling those types of files in about:plugins.
If an application other than Windows Media Player is the default player for Windows Media formats and you receive an error message or the file doesn't play when you click on a link to one of those file types, either restore Windows Media Player as the default player for those file types or change your Firefox options for Download Actions to open them with the default (or another) application, instead of the "Use this Plug-In" Windows Media Player plugin option. 
A reported issue exists in certain situations where the Flash Player is used in conjunction with Firefox, whereby Shockwave Flash files will not run automatically using the plugin when opened directly in the browser, but instead will cause the "Opening" dialog box to be displayed, asking the user to choose whether to save the file or open it in a suitable application. (If this occurs, there is no way of instructing the browser to run the file directly. Contrary to advice commonly given to users experiencing this problem, the bug has nothing to do with the MIME type served with the Flash file, nor with the browser settings for Flash files.) This issue may affect users who had installed the Flash Player in a previous Firefox version and later upgraded Firefox, without first uninstalling this plugin. To resolve the problem, Firefox users should uninstall their current version of Flash Player and then reinstall it, either manually or by invoking the plugin by visiting a Web page containing an embedded Flash file and then following the browser prompts. [